6cc17f9a3f
module interfaces. This is meant as an exercise and an example to all team members. I fixed a minor shadowing issue in [x_list.ml].
847 lines
25 KiB
OCaml
847 lines
25 KiB
OCaml
(** Trace tutorial
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This module guides the reader through the writing of a simplified
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version of the trace monad [result], and the definition of a few
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operations that make it easier to work with [result].
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*)
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module Trace_tutorial = struct
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(** The trace monad is fairly similar to the predefined option
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type. *)
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type annotation = string
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type error = string
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(** The type ['a result] is used by the trace monad to both model an
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expected value of type ['a] or the failure to obtain it, instead
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of working directly with ['a] values and handling separately
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errors, for example by means of exceptions. (See the type [('a,'b)
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result] in the module [Pervasives] of the OCaml system for a
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comparable approach to error handling.)
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The type ['a result] carries either a value of type ['a], with a
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list of annotations (information about past successful
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computations), or it is a list of errors accumulated so far.
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The former case is denoted by the data constructor [Ok], and the
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second by [Errors].
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*)
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type 'a result =
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Ok of 'a * annotation list
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| Errors of error list
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(** The function [divide_trace] shows the basic use of the trace
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monad.
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*)
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let divide_trace a b =
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if b = 0
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then Errors [Printf.sprintf "division by zero: %d/%d" a b]
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else Ok (a/b, [])
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(** The function [divide_three] shows that when composing two
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functions, if the first call fails, the error is passed along
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and the second call is not evaluated. (A pattern called
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"error-passing style").
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*)
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let divide_three a b c =
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match divide_trace a b with
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Ok (a_div_b , _) -> divide_trace a_div_b c
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| errors -> errors
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(** The function [divide_three_annot] shows that when composing two
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functions, if both calls are successful, the lists of
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annotations are joined.
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*)
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let divide_three_annot a b c =
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match divide_trace a b with
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Ok (a_div_b, annot1) -> (
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match divide_trace a_div_b c with
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Ok (a_div_b_div_c, annot2) ->
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Ok (a_div_b_div_c, annot2 @ annot1)
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| errors -> errors)
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| errors -> errors
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(** The systematic matching of the result of each call in a function
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composition is bulky, so we define a [bind] function which takes
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a function [f: 'a -> 'b result] and applies it to a current ['a
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result] (not ['a]).
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{ul
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{li If the current result is an error, then [bind]
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returns that same error without calling [f];}
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{li otherwise [bind] unwraps the [Ok] of the current result
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and calls [f] on it:
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{ul
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{li That call itself may return an error;}
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{li if not, [bind] combines the annotations and returns the last
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result.}}}}
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*)
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let bind (f: 'a -> 'b result) : 'a result -> 'b result =
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function
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Ok (x, annot) -> (
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match f x with
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Ok (x', annot') -> Ok (x', annot' @ annot)
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| errors -> ignore annot; errors)
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| Errors _ as e -> e
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(** The function [divide_three_bind] is equivalent to the verbose
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[divide_three] above, but makes use of [bind].
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*)
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let divide_three_bind a b c =
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let maybe_a_div_b = divide_trace a b in
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let continuation a_div_b = divide_trace a_div_b c
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in bind continuation maybe_a_div_b
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(** The operator [(>>?)] is a redefinition of [bind] that makes the
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program shorter, at the cost of a slightly
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awkward reading because the two parameters are swapped.
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*)
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let (>>?) x f = bind f x
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(** The function [divide_three_bind_symbol] is equivalent to
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[divide_three_bind], but makes use of the operator [(>>?)].
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*)
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let divide_three_bind_symbol a b c =
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let maybe_a_div_b = divide_trace a b in
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let continuation a_div_b = divide_trace a_div_b c in
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maybe_a_div_b >>? continuation
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(** The function [divide_three_bind_symbol'] is equivalent to
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[divide_three_bind_symbol], where the two temporary [let]
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definitions are inlined for a more compact reading.
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*)
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let divide_three_bind_symbol' a b c =
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divide_trace a b >>? (fun a_div_b -> divide_trace a_div_b c)
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(** This is now fairly legible, but chaining many such functions is
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not the usual way of writing code. We use the PPX extension to
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the OCaml compiler [ppx_let] to add some syntactic sugar.
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The extension framework PPX is enabled by adding the following
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lines inside the section [(library ...)] or [(executable ...)]
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of the [dune] file for the project that uses [ppx_let], like so:
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[(preprocess
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(pps simple-utils.ppx_let_generalized))]
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The extension [ppx_let] requires the module [Let_syntax] to be
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defined.
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*)
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module Let_syntax = struct
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let bind m ~f = m >>? f
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module Open_on_rhs_bind = struct end
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end
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(** The function [divide_three_bind_ppx_let] is equivalent to the
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function [divide_three_bind_symbol']. The only difference is
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that the module [Open_on_rhs_bind] is implicitly opened around
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the expression on the righ-hand side of the [=] sign, namely
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[divide_trace a b].
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*)
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let divide_three_bind_ppx_let a b c =
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let%bind a_div_b = divide_trace a b
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in divide_trace a_div_b c
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(** The function [divide_many_bind_ppx_let] shows how well this
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notation composes.
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*)
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let divide_many_bind_ppx_let a b c d e f =
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let x = a in
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let%bind x = divide_trace x b in
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let%bind x = divide_trace x c in
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let%bind x = divide_trace x d in
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let%bind x = divide_trace x e in
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let%bind x = divide_trace x f
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in Ok (x, [])
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(** The function [ok] is a shorthand for an [Ok] without
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annotations.
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*)
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let ok x = Ok (x, [])
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(** The function [map] lifts a regular ['a -> 'b] function on values to
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a function on results, of type ['a result -> 'b result].
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*)
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let map f = function
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Ok (x, annotations) -> Ok (f x, annotations)
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| e -> e
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(** The function [bind_list] turns a list of results of type [('a
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result) list] into a result of list, of type [('a list) result],
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as follows.
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{ul
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{li If the list only contains [Ok] values, it strips the [Ok]
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of each element and returns that list wrapped with [Ok].}
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{li Otherwise, one or more of the elements of the input list
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is [Errors], then [bind_list] returns the first error in the
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list.}}
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*)
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let rec bind_list = function
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[] -> ok []
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| hd::tl ->
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hd >>? fun hd ->
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bind_list tl >>? fun tl ->
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ok @@ hd::tl
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(** A major feature of [Trace] is that it enables having a stack of
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errors (that should act as a simplified stack frame), rather
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than a unique error. It is done by using the function
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[trace]. For instance, let's say that you have a function that
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can trigger two errors, and you want to pass their data along
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with an other error, what you would usually do is:
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[let foobarer ... = ... in
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let value =
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try get key map with
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Bad_key _ -> raise (Error ("bad key", key, map))
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| Missing_value _ -> raise (Error ("missing index", key, map))
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in ...]
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With [Trace], you would instead:
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[let foobarer ... = ... in
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let%bind value =
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trace (simple_error "error getting key") @@
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get key map
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in ...]
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And this will pass along the error triggered by [get key map].
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*)
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let trace err = function
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Errors e -> Errors (err::e)
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| ok -> ok
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(** The real trace monad is very similar to the one that we have
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defined above. The main difference is that the errors and
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annotations are structured data (instead of plain strings) and
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are generated lazily.
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*)
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let the_end = "End of the tutorial."
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end (* end Trace_tutorial. *)
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module J = Yojson.Basic
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module JSON_string_utils = struct
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let member n x =
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match x with
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`Null -> `Null
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| x -> J.Util.member n x
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let string = J.Util.to_string_option
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let to_list_option = fun x ->
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try Some (J.Util.to_list x) with _ -> None
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let to_assoc_option = fun x ->
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try Some (J.Util.to_assoc x) with _ -> None
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let list = to_list_option
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let assoc = to_assoc_option
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let int = J.Util.to_int_option
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let patch j k v =
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match assoc j with
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None -> j
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| Some assoc -> `Assoc (
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List.map (fun (k', v') -> (k', if k = k' then v else v')) assoc
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)
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let swap f l r = f r l
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let unit x = Some x
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let bind f = function None -> None | Some x -> Some (f x)
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let bind2 f l r =
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match l, r with
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None, None -> None
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| None, Some _ -> None
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| Some _, None -> None
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| Some l, Some r -> Some (f l r)
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let default d = function
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Some x -> x
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| None -> d
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let string_of_int = bind string_of_int
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let (||) l r = l |> default r
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let (|^) = bind2 (^)
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end
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type 'a thunk = unit -> 'a
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(** Errors are encoded in JSON. This is because different libraries
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will implement their own helpers, and we do not want to hardcode
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in their type how they are supposed to interact.
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*)
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type error = J.t
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(** Thunks are used because computing some errors can be costly, and
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we do not want to spend most of our time building errors. Instead,
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their computation is deferred.
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*)
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type error_thunk = error thunk
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(** Annotations should be used in debug mode to aggregate information
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about some value history. Where it was produced, when it was
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modified, etc. It is currently not being used. *)
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type annotation = J.t
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(** Even in debug mode, building annotations can be quite
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resource-intensive. Instead, a thunk is passed, that is computed
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only when debug information is queried (typically before a print).
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*)
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type annotation_thunk = annotation thunk
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(** Types of traced elements. It might be good to rename it [trace] at
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some point.
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*)
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type 'a result =
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Ok of 'a * annotation_thunk list
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| Error of error_thunk
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(** {1 Constructors} *)
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let ok x = Ok (x, [])
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let fail err = Error err
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(** {1 Monadic operators} *)
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let bind f = function
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Ok (x, ann) -> (
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match f x with
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Ok (x', ann') -> Ok (x', ann' @ ann)
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| Error _ as e' -> ignore ann; e')
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| Error _ as e -> e
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let map f = function
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Ok (x, annotations) -> Ok (f x, annotations)
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| Error _ as e -> e
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(** The lexical convention usually adopted for the bind function is
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[>>=], but ours comes from the Tezos code base, where the [result]
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bind is [>>?], and [Lwt]'s (threading library) is [>>=], and the
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combination of both is [>>=?].
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*)
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let (>>?) x f = bind f x
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let (>>|?) x f = map f x
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(**
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Used by PPX_let, an OCaml preprocessor.
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What it does is that, when you only care about the case where a result isn't
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an error, instead of writing:
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[
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(* Stuff that might return an error *) >>? fun ok_value ->
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(* Stuff being done on the result *)
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]
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You can write:
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[
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let%bind ok_value = (* Stuff that might return an error *) in
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(* Stuff being done on the result *)
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]
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This is much more typical of OCaml. This makes the code more
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readable, easy to write and refactor. It is used pervasively in
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LIGO.
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*)
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module Let_syntax = struct
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let bind m ~f = m >>? f
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module Open_on_rhs_bind = struct end
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end
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(** Build a thunk from a constant.
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*)
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let thunk x () = x
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(** Build a standard error, with a title, a message, an error code and
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some data.
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*)
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let mk_error
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?(error_code : int thunk option) ?(message : string thunk option)
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?(data : (string * string thunk) list option)
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?(children = []) ?(infos = [])
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~(title : string thunk) () : error =
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let error_code' = X_option.map (fun x -> ("error_code" , `Int (x ()))) error_code in
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let title' = X_option.some ("title" , `String (title ())) in
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let data' =
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let aux (key , value) = (key , `String (value ())) in
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X_option.map (fun x -> ("data" , `Assoc (List.map aux x))) data in
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let message' = X_option.map (fun x -> ("message" , `String (x ()))) message in
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let type' = Some ("type" , `String "error") in
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let children' = Some ("children" , `List children) in
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let infos' = Some ("infos" , `List infos) in
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`Assoc (X_option.collapse_list [ error_code' ; title' ; message' ; data' ; type' ; children' ; infos' ])
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let error ?data ?error_code ?children ?infos title message () = mk_error ?data ?error_code ?children ?infos ~title:(title) ~message:(message) ()
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let prepend_child = fun child err ->
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let open JSON_string_utils in
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let children_opt = err |> member "children" |> list in
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let children = match children_opt with
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| Some children -> (child ()) :: children
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| None -> [ child () ] in
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patch err "children" (`List children)
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let patch_children = fun children err ->
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let open JSON_string_utils in
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patch err "children" (`List (List.map (fun f -> f ()) children))
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(**
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Build a standard info, with a title, a message, an info code and some data.
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*)
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let mk_info
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?(info_code : int thunk option) ?(message : string thunk option)
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?(data : (string * string thunk) list option)
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~(title : string thunk) () : error =
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let error_code' = X_option.map (fun x -> ("error_code" , `Int (x ()))) info_code in
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let title' = X_option.some ("title" , `String (title ())) in
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let data' =
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let aux (key , value) = (key , `String (value ())) in
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X_option.map (fun x -> ("data" , `Assoc (List.map aux x))) data in
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let message' = X_option.map (fun x -> ("message" , `String (x ()))) message in
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let type' = Some ("type" , `String "info") in
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`Assoc (X_option.collapse_list [ error_code' ; title' ; message' ; data' ; type' ])
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let info ?data ?info_code title message () = mk_info ?data ?info_code ~title:(title) ~message:(message) ()
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let prepend_info = fun info err ->
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let open JSON_string_utils in
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let infos_opt = err |> member "infos" |> list in
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let infos = match infos_opt with
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| Some infos -> info :: infos
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| None -> [ info ] in
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patch err "infos" (`List infos)
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(** Helpers that ideally should not be used in production.
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*)
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let simple_error str () = mk_error ~title:(thunk str) ()
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let simple_info str () = mk_info ~title:(thunk str) ()
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let simple_fail str = fail @@ simple_error str
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let internal_assertion_failure str = simple_error ("assertion failed: " ^ str)
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(** To be used when you only want to signal an error. It can be useful
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when followed by [trace_strong].
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*)
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let dummy_fail = simple_fail "dummy"
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let trace info = function
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Ok _ as o -> o
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| Error err -> Error (fun () -> prepend_info (info ()) (err ()))
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(** Erase the current error stack, and replace it by the given
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error. It's useful when using [Assert] and you want to discard its
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autogenerated message.
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*)
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let trace_strong err = function
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Ok _ as o -> o
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| Error _ -> Error err
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(**
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Sometimes, when you have a list of potentially erroneous elements, you need
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to retrieve all the errors, instead of just the first one. In that case, do:
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[let type_list lst =
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let%bind lst' =
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trace_list (simple_error "Error while typing a list") @@
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List.map type_element lst in
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...]
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Where before you would have written:
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[let type_list lst =
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let%bind lst' = bind_map_list type_element lst in
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...]
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*)
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let trace_list err lst =
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let oks =
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let aux = function
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| Ok (x , _) -> Some x
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| _ -> None in
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X_list.filter_map aux lst in
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let errs =
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let aux = function
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| Error x -> Some x
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| _ -> None in
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X_list.filter_map aux lst in
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match errs with
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| [] -> ok oks
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| errs -> fail (fun () -> patch_children errs err)
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(**
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Trace, but with an error which generation may itself fail.
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*)
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let trace_r err_thunk_may_fail = function
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| Ok _ as o -> o
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| Error _ -> (
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match err_thunk_may_fail () with
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| Ok (err, annotations) -> ignore annotations; Error (err)
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| Error errors_while_generating_error ->
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(* TODO: the complexity could be O(n*n) in the worst case,
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this should use some catenable lists. *)
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Error (errors_while_generating_error)
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)
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(**
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`trace_f f error` yields a function that acts the same as `f`, but with an
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error frame that has one more error.
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*)
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let trace_f f error x =
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trace error @@ f x
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(**
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Same, but for functions with 2 parameters.
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*)
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let trace_f_2 f error x y =
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trace error @@ f x y
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(**
|
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Same, but with a prototypical error.
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*)
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let trace_f_ez f name =
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trace_f f (error (thunk "in function") name)
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let trace_f_2_ez f name =
|
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trace_f_2 f (error (thunk "in function") name)
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(**
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Check if there is no error. Useful for tests.
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*)
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let to_bool = function
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| Ok _ -> true
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| Error _ -> false
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let to_option = function
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| Ok (o, annotations) -> ignore annotations; Some o
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| Error _ -> None
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(**
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Convert an option to a result, with a given error if the parameter is None.
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*)
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let trace_option error = function
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| None -> fail error
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| Some s -> ok s
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(** Utilities to interact with other data-structure. [bind_t] takes
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an ['a result t] and makes a ['a t result] out of it. It "lifts" the
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error out of the type. The most common context is when mapping a
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given type. For instance, if you use a function that can fail in
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[List.map], you need to manage a whole list of results. Instead,
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you do [let%bind lst' = bind_list @@ List.map f lst], which will
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yield an ['a list]. [bind_map_t] is roughly syntactic sugar for
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[bind_t @@ T.map]. So that you can rewrite the previous example as
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[let%bind lst' = bind_map_list f lst]. Same thing with folds.
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*)
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let bind_map_option f = function
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| None -> ok None
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| Some s -> f s >>? fun x -> ok (Some x)
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let rec bind_list = function
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| [] -> ok []
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| hd :: tl -> (
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hd >>? fun hd ->
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bind_list tl >>? fun tl ->
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ok @@ hd :: tl
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)
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let bind_ne_list = fun (hd , tl) ->
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hd >>? fun hd ->
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bind_list tl >>? fun tl ->
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ok @@ (hd , tl)
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let bind_smap (s:_ X_map.String.t) =
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let open X_map.String in
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let aux k v prev =
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prev >>? fun prev' ->
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v >>? fun v' ->
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ok @@ add k v' prev' in
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fold aux s (ok empty)
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let bind_fold_smap f init (smap : _ X_map.String.t) =
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let aux k v prev =
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prev >>? fun prev' -> f prev' k v
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in X_map.String.fold aux smap init
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let bind_map_smap f smap = bind_smap (X_map.String.map f smap)
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let bind_concat (l1:'a list result) (l2: 'a list result) =
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let%bind l1' = l1 in
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let%bind l2' = l2 in
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ok @@ (l1' @ l2')
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let bind_map_list f lst = bind_list (List.map f lst)
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let rec bind_map_list_seq f lst = match lst with
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| [] -> ok []
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| hd :: tl -> (
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let%bind hd' = f hd in
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let%bind tl' = bind_map_list_seq f tl in
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ok (hd' :: tl')
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)
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let bind_map_ne_list : _ -> 'a X_list.Ne.t -> 'b X_list.Ne.t result =
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fun f lst -> bind_ne_list (X_list.Ne.map f lst)
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let bind_iter_list : (_ -> unit result) -> _ list -> unit result =
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fun f lst -> bind_map_list f lst >>? fun _ -> ok ()
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let bind_location (x:_ Location.wrap) =
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x.wrap_content >>? fun wrap_content ->
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ok { x with wrap_content }
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let bind_map_location f x = bind_location (Location.map f x)
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let bind_fold_list f init lst =
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let aux x y =
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x >>? fun x ->
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f x y
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in
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List.fold_left aux (ok init) lst
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module TMap(X : Map.OrderedType) = struct
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module MX = Map.Make(X)
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let bind_fold_Map f init map =
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let aux k v x =
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x >>? fun x ->
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f ~x ~k ~v
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in
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MX.fold aux map (ok init)
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let bind_map_Map f map =
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let aux k v map' =
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map' >>? fun map' ->
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f ~k ~v >>? fun v' ->
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ok @@ MX.update k (function
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| None -> Some v'
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| Some _ -> failwith "key collision, shouldn't happen in bind_map_Map")
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map'
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in
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MX.fold aux map (ok MX.empty)
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end
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let bind_fold_pair f init (a,b) =
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let aux x y =
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x >>? fun x ->
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f x y
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in
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List.fold_left aux (ok init) [a;b]
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let bind_fold_triple f init (a,b,c) =
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let aux x y =
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x >>? fun x ->
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f x y
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in
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List.fold_left aux (ok init) [a;b;c]
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let bind_fold_map_list = fun f acc lst ->
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let rec aux (acc , prev) f = function
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| [] -> ok (acc , prev)
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| hd :: tl ->
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f acc hd >>? fun (acc' , hd') ->
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aux (acc' , hd' :: prev) f tl
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in
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aux (acc , []) f lst >>? fun (acc' , lst') ->
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ok @@ (acc' , List.rev lst')
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let bind_fold_map_right_list = fun f acc lst ->
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let rec aux (acc , prev) f = function
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| [] -> ok (acc , prev)
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| hd :: tl ->
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f acc hd >>? fun (acc' , hd') ->
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aux (acc' , hd' :: prev) f tl
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in
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aux (acc , []) f (List.rev lst) >>? fun (_acc' , lst') ->
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ok lst'
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let bind_fold_right_list f init lst =
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let aux x y =
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x >>? fun x ->
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f x y
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in
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X_list.fold_right' aux (ok init) lst
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let bind_find_map_list error f lst =
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let rec aux lst =
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match lst with
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| [] -> fail error
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| hd :: tl -> (
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match f hd with
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| Error _ -> aux tl
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| o -> o
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)
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in
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aux lst
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let bind_list_iter f lst =
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let aux () y = f y in
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bind_fold_list aux () lst
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let bind_or (a, b) =
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match a with
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| Ok _ as o -> o
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| _ -> b
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let bind_map_or (fa , fb) c =
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bind_or (fa c , fb c)
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let bind_lr (type a b) ((a : a result), (b:b result)) : [`Left of a | `Right of b] result =
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match (a, b) with
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| (Ok _ as o), _ -> map (fun x -> `Left x) o
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| _, (Ok _ as o) -> map (fun x -> `Right x) o
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| _, Error b -> Error b
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let bind_lr_lazy (type a b) ((a : a result), (b:unit -> b result)) : [`Left of a | `Right of b] result =
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match a with
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| Ok _ as o -> map (fun x -> `Left x) o
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| _ -> (
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match b() with
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| Ok _ as o -> map (fun x -> `Right x) o
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| Error b -> Error b
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)
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let bind_and (a, b) =
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a >>? fun a ->
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b >>? fun b ->
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ok (a, b)
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let bind_and3 (a, b, c) =
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a >>? fun a ->
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b >>? fun b ->
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c >>? fun c ->
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ok (a, b, c)
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let bind_pair = bind_and
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let bind_map_pair f (a, b) =
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bind_pair (f a, f b)
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let bind_fold_map_pair f acc (a, b) =
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f acc a >>? fun (acc' , a') ->
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f acc' b >>? fun (acc'' , b') ->
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ok (acc'' , (a' , b'))
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let bind_map_triple f (a, b, c) =
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bind_and3 (f a, f b, f c)
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(**
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Wraps a call that might trigger an exception in a result.
|
|
*)
|
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let generic_try err f =
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try (
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ok @@ f ()
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) with _ -> fail err
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|
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(**
|
|
Same, but with a handler that generates an error based on the exception,
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rather than a fixed error.
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*)
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let specific_try handler f =
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try (
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ok @@ f ()
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) with exn -> fail (handler exn)
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(**
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Same, but tailored to `Sys_error`s, found in `Sys` from `Pervasives`.
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*)
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let sys_try f =
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let handler = function
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| Sys_error str -> error (thunk "Sys_error") (fun () -> str)
|
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| exn -> raise exn
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in
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specific_try handler f
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|
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(**
|
|
Same, but for a given command.
|
|
*)
|
|
let sys_command command =
|
|
sys_try (fun () -> Sys.command command) >>? function
|
|
| 0 -> ok ()
|
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| n -> fail (fun () -> error (thunk "Nonzero return code") (fun () -> (string_of_int n)) ())
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|
|
(**
|
|
Assertion module.
|
|
Would make sense to move it outside Trace.
|
|
*)
|
|
module Assert = struct
|
|
let assert_fail ?(msg="didn't fail") = function
|
|
| Ok _ -> simple_fail msg
|
|
| _ -> ok ()
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let assert_true ?(msg="not true") = function
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| true -> ok ()
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| false -> simple_fail msg
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let assert_equal ?msg expected actual =
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assert_true ?msg (expected = actual)
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let assert_equal_string ?msg expected actual =
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let msg =
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let default = Format.asprintf "Not equal string : expected \"%s\", got \"%s\"" expected actual in
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X_option.unopt ~default msg in
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assert_equal ~msg expected actual
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let assert_equal_int ?msg expected actual =
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let msg =
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let default = Format.asprintf "Not equal int : expected %d, got %d" expected actual in
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X_option.unopt ~default msg in
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assert_equal ~msg expected actual
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let assert_equal_bool ?msg expected actual =
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let msg =
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let default = Format.asprintf "Not equal bool : expected %b, got %b" expected actual in
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X_option.unopt ~default msg in
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assert_equal ~msg expected actual
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let assert_none ?(msg="not a none") opt = match opt with
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| None -> ok ()
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| _ -> simple_fail msg
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let assert_list_size ?(msg="lst doesn't have the right size") lst n =
|
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assert_true ~msg List.(length lst = n)
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let assert_list_empty ?(msg="lst isn't empty") lst =
|
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assert_true ~msg List.(length lst = 0)
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let assert_list_same_size ?(msg="lists don't have same size") a b =
|
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assert_true ~msg List.(length a = length b)
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let assert_list_size_2 ~msg = function
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| [a;b] -> ok (a, b)
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| _ -> simple_fail msg
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let assert_list_size_1 ~msg = function
|
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| [a] -> ok a
|
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| _ -> simple_fail msg
|
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end
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let json_of_error = J.to_string
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|
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let error_pp out (e : error) =
|
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let open JSON_string_utils in
|
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let message =
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let opt = e |> member "message" |> string in
|
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X_option.unopt ~default:"" opt in
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let error_code =
|
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let error_code = e |> member "error_code" in
|
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match error_code with
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| `Null -> ""
|
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| _ -> " (" ^ (J.to_string error_code) ^ ")" in
|
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let title =
|
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let opt = e |> member "title" |> string in
|
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X_option.unopt ~default:"" opt in
|
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let data =
|
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let data = e |> member "data" in
|
|
match data with
|
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| `Null -> ""
|
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| _ -> J.to_string data in
|
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Format.fprintf out "%s (%s): %s. %s" title error_code message data
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let error_pp_short out (e : error) =
|
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let open JSON_string_utils in
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let title = e |> member "title" |> string || "(no title)" in
|
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let error_code = e |> member "error_code" |> int |> string_of_int || "no error code" in
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Format.fprintf out "%s (%s)" title error_code
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let errors_pp =
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Format.pp_print_list
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~pp_sep:Format.pp_print_newline
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error_pp
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let errors_pp_short =
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Format.pp_print_list
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~pp_sep:Format.pp_print_newline
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error_pp_short
|