[Haskell-cafe] testing if values can be applied to polymorphic functions

alex alex at slab.org
Mon Oct 22 11:24:50 CEST 2012

Hi all,

A while ago I made an unusual visual front-end for Haskell:

It applies functions that are both proximal in Euclidean space, and
type-compatible.  It 'compiles' to Haskell, piped into ghci, but I
(probably unwisely) wrote it in C, and ended up having to implement
some crazed version of the Haskell type system there to get it

Now I have idea for making this more interesting and practical (for
live music-making, if nothing else), and would like to re-write it all
in Haskell.  Testing type compatibility of expressions is foxing me

For example, say I have (++) and [1,2,3].  I then want to see if a
value of type (Num a => [a]) can be applied to a function of type ([a]
-> [a] -> [a]).  I've been looking at Data.Typeable.funResultTy to do
this, e.g.:

funResultTy (typeOf ((++) :: [a] -> [a] -> [a])) (typeOf ([1,2,3] ::
Num a => [a]))

But I get this:
Ambiguous type variable `a0' in the constraint:
      (Typeable a0) arising from a use of `typeOf'

Being a bit more specific doesn't help:

funResultTy (typeOf ((++) :: Typeable a => [a] -> [a] -> [a])) (typeOf
([1,2,3] :: (Typeable a, Num a) => [a]))

I guess funResultTy doesn't work when polymorphism is involved..

Perhaps I could just use the ghc-as-a-library stuff to parse and
typecheck code - would that be the way forward?

Any pointers much appreciated!

Best wishes



More information about the Haskell-Cafe mailing list