On 29/03/2007, at 11:38, Mike Hamburg wrote:
Is there any way to use RULES substitutions with type classes?
I'm writing a reactive programming arrow (same idea as Yampa,
different
design goals), and it would help performance (and not just in the
speed
sense) to be able to tell when a value derived with arr hasn't
changed.
So I'd like to be able to specialize arr to functions whose result
is an
instance of Eq.
I tried
{-# RULES "reactiveArr/Eq" reactiveArr = reactiveArrEq #-}
but got the message
Control/Arrow/Reactive/Reactive.hs:89:41:
No instance for (Eq b)
arising from instantiating a type signature
at Control/Arrow/Reactive/Reactive.hs:89:41-89
Possible fix: add (Eq b) to the tcRule
When checking the transformation rule "reactiveArr/Eq"
I tried adding various sorts of type signatures, but I couldn't
find any
way around this... is it a restriction in the RULES rewrite
engine? Is
there a workaround, or some mechanism other than RULES that I
should be
using? I could write a special "arrEq" function, but I'd like to
minimize the number of extraneous operations outside the Arrow class.
I thought this could be done already. In ghc 6.7:
\begin{code}
{-# RULES "hello/helloBool" hello = helloBool #-}
{-# RULES "hello/helloEq" forall (x::Eq a=>a) . hello x = helloEq x #-}
{-# RULES "hello/helloF" forall (f::Eq b=>a->b) . hello f = helloF f #-}
data O = O
hello _ = "hello"
helloBool :: Bool -> String
helloBool _ = "hello bool"
helloEq :: Eq a => a -> String
helloEq _ = "hello Eq"
helloF :: Eq b => (a->b) -> String
helloF _ = "hello F"
f :: Eq b => String -> b
f = undefined
normal = hello O
bool = hello False
char = hello 'a'feq :: String
feq = hello (f::String -> Bool)
\end{code}
pep:~/code/snippets$ ghc -O -fglasgow-exts -c rules.hs
pep:~/code/snippets$ ghci rules.hs
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 6.7.20070303, for
Haskell 98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Loading package base ... linking ... done.
[1 of 1] Skipping Rules ( rules.hs, rules.o )
Ok, modules loaded: Rules.
Prelude Rules> normal
"hello"
Prelude Rules> bool
"hello bool"
Prelude Rules> char
"hello Eq"
Prelude Rules> feq
"hello F"
Is that what you wanted?
pepe
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