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signature and 'a generic types
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Date: -- (:)
From: Damien Bobillot <damien.bobillot@m...>
Subject: signature and 'a generic types

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Hello,

Look at the following code :

     module Module : (sig val func : 'a -> 'b end) =
     struct let func a = a end

Here Module.func's type is 'a -> 'a. According to me 'a -> 'a is a  
subtype of 'a -> 'b, it's 'a -> 'b with 'a = 'b. Nevertheless ocamlc  
give the following error :

     Values do not match:
       val func : 'a -> 'a
     is not included in
       val func : 'a -> 'b

Why ?

I also try to use the reverse match :

     module Module : (sig val func : 'a -> 'a end) = struct
         let l = ref []
         let func a = List.assoc a !l
     end

Where Module.func is of type 'a -> 'b. Ocamlc also give me an error,  
but I completely understand it here : 'a -> 'b is really not a  
subtype of 'a -> 'a.

In this case I also don't understand why neither " 'a -> 'b is  
included in 'a -> 'a " nor " 'a -> 'a is included in 'a -> 'b " are  
true. To my mind, one should be true.

Is it a caml limitation/bug, or am I wrong somewhere ?

-- 
Damien Bobillot


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