[Haskell-cafe] know a workaround for greedy context reduction?

Nicolas Frisby nicolas.frisby at gmail.com
Sun Dec 7 17:57:32 EST 2008

Seems I got ahead of myself with the bug search. I was thinking bug
because when I ascribe a type, I expect the compiler to check and then
respect it. With the "most general type" specification of the ":type"
command in mind, this does make sense. Thanks for improving my
internal notion of ":type".

My grumble may seem more legitimate from a library perspective. I
implement a type-level function Append with three (preferably hidden)
ancillary classes and a single instance in order to support the
multiple modalities (in the Mercury sense) of the Append logic
function. When a user defines another function that uses the append
method, it's obfuscating for the user to see the internal classes in
the inferred type. That's what I would like to workaround.

If we consider class C the internal and consider class D and the
function f the library's exposed interface, then I'd like to see C
instead of D in the context of f and any function the user defines
with f, especially when I have supplied a preferred type for f.

> f :: D a => () -> a
> f () = d

> *> :t f
> f :: (C a) => () -> a

No dice?

Thanks again,

On Sun, Dec 7, 2008 at 2:34 PM, Simon Peyton-Jones
<simonpj at microsoft.com> wrote:
> This is perfectly reasonable behavior I'm afraid.  If you do ":info d" you'll get d's original type signature.  But ":type" takes an *arbitrary expression* (in this case a single variable 'd', and figures out its most general type.  You could have said ":t (3*3)" for example.
> In this case, when inferring the most general type of the expression "d", GHC tries to simplify the context (D a), and uses the instance declaration to reduce it to (C a).  And then it can't simplify it further.  But you *might* have had
>        instance C a
> somewhere, in which case it'd have been able to simplify the (C a) away.  So GHC must try that route.  If it fails, you want it to "back up" to a notationally more convenient type, but GHC can't do that, I'm afraid
> Simon
> | -----Original Message-----
> | From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-
> | bounces at haskell.org] On Behalf Of Nicolas Frisby
> | Sent: 06 December 2008 03:23
> | To: haskell Cafe
> | Subject: [Haskell-cafe] know a workaround for greedy context reduction?
> |
> | With these three declarations
> |
> |   {-# LANGUAGE FlexibleInstances #-}
> |   {-# LANGUAGE UndecidableInstances #-}
> |
> |   class C a where c :: a
> |   class C a => D a where d :: a
> |   instance C a => D a where d = c
> |
> | ghci exhibits this behavior:
> |
> |   *> :t d
> |   d :: (C a) => a
> |
> | Where I would prefer "d :: (D a) => a". In my actual examples, the
> | context is much larger and I can't involve overlapping instances. Is
> | there a known workaround? I didn't find a related bug on the GHC trac,
> | and I don't know if other compilers behave in the same way.
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