Using Core.Std.List.fold_left without label

Question

I am experimenting with Core\'s List.fold_left.

# List.fold_left;;
- : \'a Core.Std.List.t -> init:\'b -> f:(\'b -> \'a -> \'b) -> \'         


        

Answer 1

@sepp2k's answer clearly explains what is happening here. Let me add one more thing.

Simply, do NOT omit labels of labeled functions. They are there for purposes. I believe no one can handle label omittion always correctly. You should always turn the warning 6 on and make it an error:

$ 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)

Answer 2

According to the manual you may only omit labels "if an application is total (omitting all optional arguments)" with the important caveat "that functions like ListLabels.fold_left whose result type is a type variable will never be considered as totally applied."

So since core's fold_left's result type is, of course, also a type variable, you can not call it without labels. What happens when you do is that the positional arguments are interpreted as arguments to the function produced by fold_left and it still waits for you to provide the labeled arguments.