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