[Haskell-cafe] asserting the type of a binding in a "do" expression

[Henning Thielemann]

On Sat, 26 Apr 2008, Ken Takusagawa wrote:

> This works (with -fglasgow-exts):
>
> foo::IO Int;
> foo = do{
>   (x::Int) <- bar;
>   return x;};
>
> bar = undefined
>
> But this does not:
>
> foo::IO a;
> foo = do{
>   (x::a) <- bar;
>   return x;};
>
> Error message: A pattern type signature cannot bind scoped type
> variables `a' unless the pattern has a rigid type context.

'asTypeOf' is the answer for Haskell98. You can also write your own helper 
functions which might simplify that for your use cases.