[Opening discussion to broader audience: I asked Simon PJ about scoped type variables and type abbreviations]
| With scoped type variables, it would be useful to be able to define | a type abbreviation in a where clause, so that the scoped | type variable could be used on a right-hand side. Apparently | this wasn't done. Was there any technical reason why not?
Would you care to give an example? Do you mean f x = e where a = Int
No. I give an example below, but I've just spent 10 minutes grubbing through the Haskell 98 Report so I can phrase the question more precisely: Given the extension to scoped type variables, I believe that the 'type' abbreviation declaration should have been 'promoted' from syntactic category 'topdecl' to syntactic category 'decl', because it is convenient and useful to be able to exploit a scoped type variable in a type abbreviation. Was this possibility simply overlooked, or was a deliberate choice made not to do it? Here is a synopsis of code I would like to write: solve :: forall m l a . Graph m l -> Fuel -> DFM a (a, Fuel) solve = solveGraph where -- in the defintions below, type variable 'a' is free -- on the RHS but is bound by the forall above type FactKont b = Graph m l -> a -> Fuel -> DFM a b type FuelKont b = Graph m l -> Fuel -> DFM a b solveMid :: m -> FactKont b -> FactKont b solveLast :: l -> FuelKont b -> FactKont b ... For my sanity I'd like to define type abbreviations that refer to 'm' and 'l' also... Under the current regime, I'm forced to do what amounts to lambda lifting on the type abbreviations. Not only does this lead to a extra type parameters which distract from the main event, but in floating the type abbreviation out to top level, I am forced to choose a unique name for it, which I might prefer not to do if the same module contains several similar functions with similar (but not identical) type abbreviations. Of course the naming issue arises even in Haskell 98, but the ability to bring type variables into scope in a 'where' clause adds urgency.
There are lots of design issues. For example, in GHC today, a scoped type variable stands for a type *variable* not for a *type*. I took the other approach initially, but I think this is better
I know this distinction is important in instance declarations. Presumably it also means that the scoped type variable can unify with something else during type inference? I fear that without seeing some formalism I can't be too sure what's going on---is there a technical report somewhere that explains the distinction? In any case, I hope this question is orthogonal to the problem of permitting a type declaration as a 'decl' in a where clause and not a mere lonely 'topdecl'. Is anybody else keen to have this ability? Norman