Jules Bean
Simon Peyton-Jones wrote: The principal difficulties here are to do with "what do we want" rather the implementation challenges.
1. Should the compiler print the type of every declaration? Should GHCi allow you to ask the type of a local decl?
IMO, ghci should definitely allow you to ask. This comes up for me every time that I write any haskell code (and in general I end up hoisting local definitions to the top level, which is a real pain if there is local scope, data or type, to hoist with it).
2. How should the variables be identified? There may be many local bindings for 'f', so you can't say just ":t f". Ditto if dumping all local bindings.
I think this is a hard question. I was imagining some kind of hierarchical system like foo.f, in the case that f is locally defined inside foo. (Yes I know we can't easily use '.' for that). There might be might be multiple fs inside the definition of foo; indeed there might even be multiple fs nested inside each other.
I just wanted to contribute a PRACTICAL TRICK I use: * If the local definition is a pattern binding f = ... then I just add f :: Ordering * If the local definition is a, say binary, function binding f p1 p2 = ... then I just add f :: Float -> Double -> Ordering (Type does not matter for the number of function arrows you need to put; only the syntactic arity of the bindings matters here.) This relies on the fact that the types Float, Double, and Ordering very rarely occur in my code --- pick your own. Now the compiler gives you wonderful error messages ``cannot match type `x y z' against Ordering'' --- so you replace ``Ordering'' with ``x y z''. If there are type variables in ``x y z'' that come from the context, take care that you have {-# LANGUAGE ScopedTypeVariables #-} at the beginning of your module (after the {-# OPTIONS ...#-} line, which should, as a matter of style, NOT contain -fglasgow-exts ) and the necessary ``forall ty1 ty2 ...'' in front of your the type in the type signature of the enclosing definition. At the cost of a re-compilation, this works wonderfully for me, and is much less painful than hoisting AND exporting local definitions. But even I still have some wishes open: * It would be nice if this worked inside the do-notation, too: do x :: Ordering x <- m (This is curently a syntax error.) * It would be nice if {-# LANGUAGE ScopedTypeVariables #-} brought in no other extensions, and if GHC would recommend the appropriate LANGUAGE pragmas instead of -fglasgow-exts when it determines that the user might have wanted some extension. (What is the right Language.Extension for GADTs?) Wolfram