Hi,
Am Montag, den 02.10.2017, 17:03 -0700 schrieb Conal Elliott:
> My questions:
>
> * Is it feasible for GHC to combine the constraints needed LHS and RHS to
> form an applicability condition?
> * Is there any way I can make the needed constraints explicit in my rewrite
> rules?
> * Are there any other work-arounds that would enable writing such
> RHS-constrained rules?
if you are fine writing one RULE per _instance_ of C, the following
works:
{-# LANGUAGE ExplicitForAll, TypeApplications #-}
{-# OPTIONS_GHC -Wall #-}
module RuleFail where
class C k where comp' :: k b c -> k a b -> k a c
instance C (->) where comp' = (.)
instance C (,) where comp' (_,a) (c,_) = (c,a)
-- Late-inlining version to enable rewriting.
comp :: C k => k b c -> k a b -> k a c
comp = comp'
{-# INLINE [0] comp #-}
morph :: forall k a b. (a -> b) -> k a b
morph _ = error "morph: undefined"
{-# NOINLINE morph #-}
{-# RULES "morph/(.)/->" forall f g. morph @(->) (g `comp` f) = morph g
`comp` morph f #-}
{-# RULES "morph/(.)/(,)" forall f g. morph @(,) (g `comp` f) =
morph g `comp` morph f #-}
Let’s look at the rules:
$ ghc -O -c -ddump-rules RuleFail.hs
==================== Tidy Core rules ====================
"morph/(.)/(,)" [ALWAYS]
forall (@ b)
(@ b1)
(@ a)
($dC :: C (->))
(f :: a -> b)
(g :: b -> b1).
morph @ (,) @ a @ b1 (comp @ (->) @ b @ b1 @ a $dC g f)
= comp
@ (,)
@ b
@ b1
@ a
$fC(,)
(morph @ (,) @ b @ b1 g)
(morph @ (,) @ a @ b f)
"morph/(.)/->" [ALWAYS]
forall (@ b)
(@ b1)
(@ a)
($dC :: C (->))
(f :: a -> b)
(g :: b -> b1).
morph @ (->) @ a @ b1 (comp @ (->) @ b @ b1 @ a $dC g f)
= comp
@ (->)
@ b
@ b1
@ a
$dC
(morph @ (->) @ b @ b1 g)
(morph @ (->) @ a @ b f)
As you can see, by specializing the rule to a specific k, GHC can
include the concrete instance dictionary (here, $fC(,)) _in the rule_
so it does not have to appear on the LHS. This is pretty much how
specialization works.
Is that a viable work-around for you? It involves boilerplate code, but
nothing that cannot be explained in the documentation. (Or maybe TH can
create such rules?)
If this idiom turns out to be useful, I wonder if there is a case for
-rules specified in type classes that get instantiated upon every
instance, e.g.
class C k where
comp' :: k b c -> k a b -> k a c
{-# RULES "morph/(.)/(,)" forall f g. morph @k (g `comp` f) = morph g
`comp` morph f #-}
Greetings,
Joachim
--
Joachim Breitner
m...@joachim-breitner.de
http://www.joachim-breitner.de/
signature.asc
Description: This is a digitally signed message part
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
