Using Core.Std.List.fold_left without shortcut
I am experimenting with Core List.fold_left
.
# List.fold_left;;
- : 'a Core.Std.List.t -> init:'b -> f:('b -> 'a -> 'b) -> 'b = <fun>
It works great when I specify the labels:
# List.fold_left [1;2;3] ~init:0 ~f:(+);;
- : int = 6
But I get a different result when I do not specify the labels:
# List.fold_left [1;2;3] 0 (+);;
- : init:(int -> (int -> int -> int) -> '_a) ->
f:((int -> (int -> int -> int) -> '_a) ->
int -> int -> (int -> int -> int) -> '_a) ->
'_a
= <fun>
And other partial applications also provide non-intuitive types. Why can I add an arbitrary number of 0s after the list argument?
# List.fold_left [1;2;3] 0;;
- : init:(int -> '_a) -> f:((int -> '_a) -> int -> int -> '_a) -> '_a = <fun>
# List.fold_left [1;2;3] 0 0;;
- : init:(int -> int -> '_a) ->
f:((int -> int -> '_a) -> int -> int -> int -> '_a) -> '_a
= <fun>
Several other functions have the same behavior:
# let app ~f ~x = f x;;
val app : f:('a -> 'b) -> x:'a -> 'b = <fun>
# app (fun x -> x + 1) 1;;
- : f:('a -> (int -> int) -> int -> 'b) -> x:'a -> 'b = <fun>
However, some labeled functions may return the expected result when applied without the labels specified. For example:
# List.map;;
- : 'a Core.Std.List.t -> f:('a -> 'b) -> 'b Core.Std.List.t = <fun>
# List.map [1;2;3] (fun x -> x + 1);;
- : int Core.Std.List.t = [2; 3; 4]
Why do some functions return non-intuitive values when applied without the specified labels, while others work as expected?
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According to the manual, you can only omit labels if the application is generic (excluding all optional arguments) "with an important caveat" that functions of a type ListLabels.fold_left
whose result type is a type variable will never be fully applied. "
So, since the result type core fold_left
is of course also a type variable, you cannot call it unlabeled. What happens when you do is that the positional arguments are interpreted as arguments to the function created fold_left
, and it is still waiting for you to supply the flagged arguments.
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@ sepp2k explains clearly what's going on here. Let me add one more thing.
Just DO NOT omit the labeled function labels. They are there for purposes. I believe no one can handle the label. You should always turn on warning 6 and make the mistake:
$ ocamlc -w +6 -warn-error +6 file.ml
File "file.ml", line 2, characters 17-18:
Warning 6: labels were omitted in the application of this function.
File "file.ml", line 1:
Error: Some fatal warnings were triggered (1 occurrences)
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