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/ ---
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