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

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.