Re: typing callcc

On Jan 29, 2005, at 10:07 PM, David Van Horn wrote:

> Perhaps this is obvious or a FAQ, but why does the following example 
> (from
> Harper, Duba, and MacQueen) result in a type error?
>
> callcc(fn k => if true then [ (throw k 3) + 1 ] else 5 :: (throw k 4))

The return value of the function given as an argument to callcc is
int list.  Thus, the value of the call to callcc is also int list.  
Thus,
the continuation captured by callcc takes an int list.  But you try
to pass an int.

> stdIn:2.1-2.71 Error: operator and operand don't agree [literal]
>   operator domain: int cont -> int
>   operand:         int cont -> int list
>   in expression:
>     callcc (fn k => if true then <exp> :: <exp> else <exp> :: <exp>)

As you see, the compiler follows this chain in the opposite direction:
You throw an int, so the continuation captured by callcc must take
an int, so the call of callcc must produce an int, but the function
that is the argument to callcc (which normally provides the result
of the call to callcc) returns an int list.

Matthias



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