On 2009-03-11, at 15:44, Markus Mottl wrote:
That's true, but unlike Haskell OCaml doesn't have mandatory types.
This means the user can't force the compiler to start out with
user-provided type declarations. The OCaml compiler will always run
type inference first and only try to unify the result with the
user-provided type declaration, i.e. when it's too late.
That is not quite true any more. For example, I changed the
type-checker a few years ago to start with the user-provided type
when typing a let rec, in order to be able to debug my large
recursive definitions. Note that I didn't do that from scrach,
I used an infrastructure that was already present for seeding the
type inference in some cases. IIRC, it is there for some object-
oriented reason.
Next time you have a type error on the wrong recursive call, try
annotating the function at its definition point.
For example, compare the error messages for:
let rec f x = g x []
and g x l =
match l with
| [] -> f "a"
| [a] -> f 1
| [a; b] -> f 2
| _ -> f 3
;;
versus:
let rec f (x : int) = g x []
and g x l =
match l with
| [] -> f "a"
| [a] -> f 1
| [a; b] -> f 2
| _ -> f 3
;;
Note that your second sentence is still right, because type annotations
are only used in this way in a limited number of cases (let rec is one
example).
-- Damien
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