Jules Bean <[EMAIL PROTECTED]> wrote,
concerning the problem of getting at the types of local definitions:
> 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
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