Simon Peyton-Jones wrote: > Conor and others are right; it's all to do with type inference. There is > nothing wrong with the program you are writing, but it's hard to design a > type inference algorithm that can figure out what you are doing. > > The culprit is that you want to instantiate a polymorphic function (here (.) > or ($) in your examples) with a higer-rank polymorphic type (the type of > runST, in this case). That requires impredicative polymorphism and while GHC > now allows that, it only allows it when it's pretty obvious what is going on > --- and sadly this case is not obvious enough.
I don't know enough type theory to suggest a specific patch, but I hope a future version of GHC can do the right thing for (.) and ($) in this situation (and possibly for other functions that simply rearrange their arguments, like flip). >From a friendliness-to-newbies point of view, these error messages are a tremendous wart. I've been on haskell-cafe for a little over three months and seen postings from three people who were tripped up by this (the first of them was myself). So I can't just tell someone who's just starting to learn Haskell that "f $ g y" is equivalent to "f (g y)"; I have to say "those two are *almost always* equivalent, but if you use $ and the compiler complains about not being able to match the expected and the inferred type and a type signature in the error message has the word 'forall', try rewriting that expression without the $ and see if it compiles". Eeeww. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
