What are use cases for F# module signature files?

Question

Are F# module signature files mostly useless, unlike OCaml, because F# doesn't have functors?

What are use cases for F# module signature files?

Answer 1

Clarification for future readers: this answer was written for the original question, before it was updated. The original question was:

Are F# signatures mostly useless, unlike OCaml, because F# doesn't have functors?

What are use cases for F# signatures?

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Though F# doesn't have OCaml modules and functors, it does have interfaces and objects, which provide similar, though a bit more limited functionality. These do require type signatures, because type inference doesn't work on them. For example:

type MyIntf =
    abstract member m : sting -> int

let f i = i.m "hello"  
// ^ error: cannot lookup member "m" on object "i" of indeterminate type

Notice that I've already needed a type annotation just to define the interface, since there is no other source of type information in the absence of an implementation for member m. But even besides that, the usage of my interface also doesn't work without an explicit type.

While it's not clear that inferring types for objects and interfaces is actually impossible, F# just doesn't attempt to do it. The official F# policy is, as long as you're purely functional, you get HM type interference, but as soon as you get into objects and members, you're on your own, the compiler won't help you.

To make the above function f work, I have to annotate the type of the i parameter:

let f (i : MyIntf) = i.m "hello"   // works now

Besides this technical requirement, type annotations are, of course, immensely valuable as documentation (which is guaranteed to not go out of sync), as well as barriers for runaway type inference.