Developer Center

So, i'am learning OCaml step by step and i've recently met the great world of "module langage" in OCaml.
Here is my problem:
module type TEST = sig
val addend : 'a -> list
end
module Test : TEST =
struct
let addend (val,lol) =
val::lol
end
when i try to use this inside the interpretor it tells me that i have a syntax error ...
I know that's this kind of anoying question asked by this guy who's a noob and thinks stack overflow is here to rewrite is poorly optimised code but still, after hours spend searching for a solution i've almost gived up at this point...
OCaml is cool, but why is it so hard.

First, in the definition of the type of your module : you have to specify the type of element of your list. You cannot use list alone.
The following code will compile :
module type TEST = sig
val addend : 'a -> 'a list
end;;
But will not do what you want as you want to define later a function that takes a element, a list and puts this element in this list.
So the signature of such function is :
'a -> 'a list -> 'a list
So the type of your module shall be :
module type TEST = sig
val addend : 'a -> 'a list -> 'a list
end;;
And the implementation :
module Test : TEST =
struct
let addend a l = a::l
end
;;
Please note that in your code, you may be influenced by C like language where you pass the arguments in parenthesis. In Ocaml, parenthesis are used for tuples ; this means that your implementation was different from the signature.

You get a syntax error becaue val is a keyword. You cannot use it as an identifier.