Why do some OCaml functions take () as a parameter?

Example in Unix m开发者_如何学编程odule:

val environment : unit -> string array

Why not just:

val environment : string array

?

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Because it denotes a function that takes a value of type unit as its parameter. The unit type is only inhabited by the value "()". This is usually used to mean that the function is going to perform some kind of IO or induce a side-effect, and needs no input. The second type signature you provided is the signature for a value, not a function that can be applied. If some expression were bound to this name, that expression would be evaluated at the time the value binding takes place, not at the time it is referenced (as is the case with function application).

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The unit () parameter is similar to a void parameter in C. It denotes a function that does not use its argument to compute something.

In your example, the environment can be changed by the program, so it cannot just be a static value computed at the beginning of the program, but at the same time, its value does not depend on its argument.

For example:

let e1 = Unix.environment ();;
Unix.putenv "USER" "somebody_else";;
let e2 = Unix.environment ();;
e1 = e2;;

And you can see that e1 and e2 are different, so Unix.environment cannot just be of type (string * string) array, but needs to be a function.

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If you were in a lazy language, such as Haskell, with no side effects and objects being evaluated only when needed, there would be no need for these dummy arguments.

In OCaml, requiring an argument of type unit (whose only value is ()) serves to freeze the computation until the argument is supplied. In this case, it freezes the computation until Unix.environment () is to be computed — which is especially important since it can have different values throughout time (again, OCaml has side effects).