Hi,
I tested the following, why does the rewrite rules not fire
when using tuples also in testRewrite2, testRewriteReverse2? compiling:
rm *.o; ghc -fglasgow-exts -ddump-simpl-stats -O9 --make rules.hs
module Main where
main :: IO ()
main = do
print $ test 0
print $ test2 0
print $ testRewrite 0
print $ testRewriteReverse 0
print $ testRewrite2 0
print $ testRewriteReverse2 0
test :: a → Bool
test x = pi
where
f = replicate 2000 x
i = repeat x
pf = lenGT f 300
pi = lenGT i 300
test2 :: a → (Bool,Bool)
test2 x = (pf,pi)
where
f = replicate 2000 x
i = repeat x
pf = lenGT f 300
pi = lenGT i 300
testRewrite :: a → Bool
testRewrite x = pi
where
f = replicate 2000 x
i = repeat x
lf = length f
li = length i
pf = lf > 300
pi = li > 300
testRewriteReverse :: a → Bool
testRewriteReverse x = pi
where
f = replicate 2000 x
i = repeat x
lf = length f
li = length i
pf = 300 <= lf
pi = 300 <= li
testRewrite2 :: a → (Bool,Bool)
testRewrite2 x = (pf,pi)
where
f = replicate 2000 x
i = repeat x
lf = length f
li = length i
pf = lf > 300
pi = li > 300
testRewriteReverse2 :: a → (Bool,Bool)
testRewriteReverse2 x = (pf,pi)
where
f = replicate 2000 x
i = repeat x
lf = length f
li = length i
pf = 300 <= lf
pi = 300 <= li
lengthOP :: (Num a, Ord a) ⇒ Bool → (a → a → Bool) → [b] → a → Bool
lengthOP v (⊜) [] n = 0 ⊜ n
lengthOP v (⊜) xxs n = co xxs 0
where
co (_:xs) c | n > c = co xs (c+1)
| otherwise = v
co [] c = c ⊜ n
lenEQ = lengthOP False (==)
lenLT = lengthOP False (<)
lenLE = lengthOP False (<=)
lenGT = lengthOP True (>)
lenGE = lengthOP True (>=)
{-# RULES
-- | length
"lenEQ_LHS" forall xs n. (length xs) == n = lenEQ xs n
"lenLT_LHS" forall xs n. (length xs) < n = lenLT xs n
"lenLE_LHS" forall xs n. (length xs) <= n = lenLE xs n
"lenGT_LHS" forall xs n. (length xs) > n = lenGT xs n
"lenGE_LHS" forall xs n. (length xs) >= n = lenGE xs n
"lenEQ_RHS" forall xs n. n == (length xs) = lenEQ xs n
"lenLT_RHS" forall xs n. n < (length xs) = lenGE xs n
"lenLE_RHS" forall xs n. n <= (length xs) = lenGT xs n
"lenGT_RHS" forall xs n. n > (length xs) = lenLE xs n
"lenGE_RHS" forall xs n. n >= (length xs) = lenLT xs n
#-}
Best Regards,
Cetin Sert
2008/12/18 Luke Palmer
<lrpalmer@gmail.com>
This does not answer your question, but you can solve this problem without rewrite rules by having length return a lazy natural:
data Nat = Zero | Succ Nat
And defining lazy comparison operators on it.
Of course you cannot replace usages of Prelude.length. But I am really not in favor of rules which change semantics, even if they only make things less strict. My argument is the following. I may come to rely on such nonstrictness as in:
bad xs = (length xs > 10, length xs > 20)
bad [1..] will return (True,True). However, if I do an obviously semantics-preserving refactor:
bad xs = (l > 10, l > 20)
where
l = length xs
My semantics are not preserved: bad [1..] = (_|_, _|_) (if/unless the compiler is clever, in which case my semantics depend on the compiler's cleverness which is even worse)
Luke
2008/12/18 Cetin Sert <cetin.sert@gmail.com>
Hi *^o^*,
With the following rewrite rules:
lengthOP :: (Num a, Ord a) ⇒ Bool → (a → a → Bool) → [b] → a → Bool
lengthOP v (⊜) [] n = 0 ⊜ n
lengthOP v (⊜) xxs n = co xxs 0
where
co [] c = c ⊜ n
co (_:xs) c | n > c = co xs (c+1)
| otherwise = v
lenEQ = lengthOP False (==)
lenLT = lengthOP False (<)
lenLE = lengthOP False (<=)
lenGT = lengthOP True (>)
lenGE = lengthOP True (>=)
{-# RULES
-- | length
"lenEQ_LHS" forall xs n. (length xs) == n = lenEQ xs n
"lenLT_LHS" forall xs n. (length xs) < n = lenLT xs n
"lenLE_LHS" forall xs n. (length xs) <= n = lenLE xs n
"lenGT_LHS" forall xs n. (length xs) > n = lenGT xs n
"lenGE_LHS" forall xs n. (length xs) >= n = lenGE xs n
"lenEQ_RHS" forall xs n. n == (length xs) = lenEQ xs n
"lenLT_RHS" forall xs n. n < (length xs) = lenGE xs n
"lenLE_RHS" forall xs n. n <= (length xs) = lenGT xs n
"lenGT_RHS" forall xs n. n > (length xs) = lenLE xs n
"lenGE_RHS" forall xs n. n >= (length xs) = lenLT xs n
-- | genericLength
"glenEQ_LHS" forall xs n. (genericLength xs) == n = lenEQ xs n
"glenLT_LHS" forall xs n. (genericLength xs) < n = lenLT xs n
"glenLE_LHS" forall xs n. (genericLength xs) <= n = lenLE xs n
"glenGT_LHS" forall xs n. (genericLength xs) > n = lenGT xs n
"glenGE_LHS" forall xs n. (genericLength xs) >= n = lenGE xs n
"glenEQ_RHS" forall xs n. n == (genericLength xs) = lenEQ xs n
"glenLT_RHS" forall xs n. n < (genericLength xs) = lenGE xs n
"glenLE_RHS" forall xs n. n <= (genericLength xs) = lenGT xs n
"glenGT_RHS" forall xs n. n > (genericLength xs) = lenLE xs n
"glenGE_RHS" forall xs n. n >= (genericLength xs) = lenLT xs n
#-}
1) Is there a way to tell where 'length' is mentioned, what is meant is for example 'Prelude.length' or any length that works on lists?
2) How can I avoid the following error messages?
module Main where
import Data.List
main :: IO Int
print $ length (repeat 0) > 200
print $ 200 < length (repeat 0)
print $ genericLength (repeat 0) > 200 -- error
print $ 200 < genericLength (repeat 0) -- error
return 0
########:
Could not deduce (Ord i) from the context (Eq i, Num i)
arising from a use of `lenEQ' at ########
Possible fix: add (Ord i) to the context of the RULE "glenEQ_LHS"
In the expression: lenEQ xs n
When checking the transformation rule "glenEQ_LHS"
########:
Could not deduce (Ord a) from the context (Eq a, Num a)
arising from a use of `lenEQ' at ########
Possible fix: add (Ord a) to the context of the RULE "glenEQ_RHS"
In the expression: lenEQ xs n
When checking the transformation rule "glenEQ_RHS"
3) What speaks for/against broad usage of such rewrite rules involving list lengths?
Best Regards,
Cetin Sert
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