Re: [Haskell-cafe] List comparisons and permutation group code

David House wrote:

On 19/10/06, Brandon Moore wrote:

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isIdentity xs = xs == takeLengthOf xs [1..] where takeLengthOf xs ys = zipWith const ys xs

You probably mean zipWith (flip const) xs ys. Either way, as long as I didn't write "zipWith const xs ys".

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for finite lists, takeLengthOf xs ys == take (length xs) ys

This ruins the laziness again:

Hugs.Base> let takeLengthOf xs ys = take (length xs) ys; isIdentity xs = xs == takeLengthOf xs [1..] in isIdentity (1:3:2:[4..10000]) False (210064 reductions, 278075 cells) Hugs.Base> let takeLengthOf = zipWith (flip const); isIdentity xs = xs == takeLengthOf xs [1..] in isIdentity (1:3:2:[4..1000]) False (60 reductions, 114 cells)

It's meant as an explanation, and partial specification: Prelude> let takeLength xs ys = zipWith const ys xs Prelude> Test.QuickCheck.test (\xs ys -> takeLength xs ys == take (length xs) ys) OK, passed 100 tests. This reminds me that QuickCheck only generates finite lists. Brandon