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I'm new to OCaml. I want to define a type that is used in one function only. I don't want to make it available outside that function. Can we define it inside that function? Or is there some other way to achieve the same?
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Function Expression Function in ocaml is a expression. That means, a function is a value that represents the function. This is also called anonymous function, lambda. (* syntax for a function expression *) fun n -> n + 1;; (* applying a function to a value *) (fun n -> n + 1) 2;; (* ⇒ 3 *)
OCaml uses this convention to help catch more type errors. Some of the built-in types are not types per se but rather families of types, or compound types, or type constructors. Tuples, lists, arrays, records, functions, objects and variants are of this sort.
ML-derived languages like OCaml are "mostly pure". They allow side-effects through things like references and arrays, but by and large most of the code you'll write will be pure functional because they encourage this thinking.
The type 'a is a type variable, and stands for any given type. The reason why sort can apply to lists of any type is that the comparisons (=, <=, etc.) are polymorphic in OCaml: they operate between any two values of the same type. This makes sort itself polymorphic over all list types.
If you have recent version of OCaml, you could use a local module:
let f x =
let module Local = struct
type t = A | B
end in
...
It may be more natural to define the type at top level and just leave it out of the .mli, though: that would keep it globally hidden, although it would be visible to other code in the file.
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