RE: [Caml-list] format strings

[David Allsopp <dra-news@metastack.com>]

Jiten Pathy wrote:
> When i define a wrapper around format functions like
> 
> let f s = (fun x -> Printf.sprintf s x);;
> 
> val f : ('a -> 'b, unit, bytes) format -> 'a -> 'b = <fun>
> 
> I am confused about the inferred type. where does this 'b comes from?

Your wrapper carries the requirement that the format [s] must have at least one %-parameter. Writing without the fun might make this more obvious:

let f s x = Printf.sprintf s x

The 'a -> 'b is the encoding of this requirement. f "foo" will type. If instead you wrote:

let f s = Printf.sprintf s

you'd get the type you're expecting: ('a, unit, bytes) format -> 'a

and you can have format strings with no %-parameters (so f "foo" now types). If you meant to have it so that the format string must have *exactly* one %-parameter then you need an annotation:

let f s x : string = Printf.sprintf s x

and now f "%d" will type, but f "%s %d" will not.

HTH,


David