apfelmus wrote:
Note that using Peano-numbers can achieve the same effect of stopping the length calculation as soon as more than one character is different.
data Nat = Zero | Succ Nat deriving (Eq, Ord)
instance Num Nat where (Succ x) + y = Succ (x+y) Zero + y = y
Very nice (and quite fast), thanks for sharing this. One point: Writing down the equations for (+) by looking at the left argument first, you introduce an additional constraint to the user program, since if we have lenL [] = 0 lenL (x:xs) = 1 + lenL xs lenR [] = 0 lenR (x:xs) = lenR xs + 1 then *FingerSpell> (lenL (repeat ()) :: Nat) < 10 False *FingerSpell> (lenR (repeat ()) :: Nat) < 10 *** Exception: stack overflow So you can change a program that returns a proper value to one that loops by replacing lenL with lenR. Such problems are very difficult to track down, even if the library documentation states it very clearly. /BR -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---