Cool! Probably one should start teaching with 'case' instead of
pattern function definitions; that would put an accent on what is
forced and in what order. Only after the student understands the
laziness issues, introduce pattern signatures.
2009/6/8 Martijn van Steenbergen
Hello,
While grading a Haskell student's work I ran into an example of a program not being lazy enough. Since it's such a basic and nice example I thought I'd share it with you:
One part of the assignment was to define append :: [a] -> [a] -> [a], another to define cycle2 :: [a] -> [a]. This was her (the majority of the students in this course is female!) solution:
append :: [a] -> [a] -> [a] append [] ys = ys append xs [] = xs append (x:xs) ys = x : (append xs ys)
cycle2 :: [a] -> [a] cycle2 [] = error "empty list" cycle2 xs = append xs (cycle2 xs)
This definition of append works fine on any non-bottom input (empty, finite, infinite), but due to the unnecessary second append case, cycle2 never produces any result.
Martijn. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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