type store is
  record
    goal     : nat;
    deadline : timestamp;
    backers  : map (address, nat);
    funded   : bool
  end

entrypoint contribute (storage store : store;
                       const sender  : address;
                       const amount  : mutez)
  : store * list (operation) is
  var operations : list (operation) := []
  begin
    if now > store.deadline then
      fail "Deadline passed"
    else
      case store.backers[sender] of
        None ->
          patch store with
            record
              backers = add_binding ((sender, amount), store.backers)
            end
      | _ -> skip
      end
  end with (store, operations)

entrypoint withdraw (storage store : store; const sender : address)
  : store * list (operation) is
  var operations : list (operation) := []
  begin
    if sender = owner then
      if now >= store.deadline then
        if balance >= store.goal then
          begin
            patch store with record funded = True end;
            operations := [Transfer (owner, balance)]
          end
        else fail "Below target"
      else fail "Too soon"
    else skip
  end with (store, operations)

entrypoint claim (storage store : store; const sender : address)
  : store * list (operation) is
  var operations : list (operation) := []
  var amount : mutez := 0
  begin
    if now <= store.deadline then
      fail "Too soon"
    else
      case store.backers[sender] of
        None ->
          fail "Not a backer"
      | Some (amount) ->
          if balance >= store.goal || store.funded then
            fail "Cannot refund"
          else
            begin
              amount := store.backers[sender];
              patch store with
                record
                  backers = remove_entry (sender, store.backers)
                end;
              operations := [Transfer (sender, amount)]
            end
      end
  end with (store, operations)