Will OCaml convert multi-argument function to currying or the other way around?

Question

When I was learning OCaml essentials, I was told that every function in OCaml is actually a function with only one parameter. A multi-argument function is actually a functio

Answer 1

As far as the semantics of the OCaml language is concerned both of those definitions are completely equivalent definitions of a curried function. There's no such thing as a multi-argument function in the semantics of the OCaml language.

However the implementation is a different matter. Specifically the current implementation of the OCaml language supports multi-argument functions in its internal representation. When a curried function is defined a certain way (i.e. as let f x y = ... or let f = fn x -> fn y -> ...), this will be compiled to a multi-argument function internally. However if it is defined differently (like let f x = (); fn y -> ... in the linked question), it will be compiled to a curried function. This is only an optimization and does not affect the semantics of the language in any way. All three ways of defining a curried function are semantically equivalent.

Regarding your specific question about what gets turned into what: Since the transformation isn't from one piece of OCaml code into another piece of OCaml code, but rather from OCaml code to an internal representation, I think the most accurate way to describe it would be to say that the OCaml compiler turns both let plus x y = x + y and let plus = fn x -> fn y -> x + y into the same thing internally, not that it turns one into the other.