Fun with Haskell view patterns

5 comments on “Fun with Haskell view patterns”

1. Joachim Breitner says:

   2009-01-11 at 06:15

   Is there a copy’n’paste error in
   Prelude> dropPrefix “foolish” “football”
   (“lish”, “ball”)
   which should rather be
   Prelude> dropPrefix “foolish” “football”
   (“lish”, “tball”)
   ?

2. Marc says:

   2009-01-11 at 07:31

   I don’t know about newer GHCs, but in 6.8.1, dropPrefix “foo” “foobar” gives a pattern match error (since left becomes [] after some time).
   Also, what is the “after” function for?

3. Alf says:

   2009-01-11 at 10:20

   Marc: presumably ‘after’ was the original name of dropPrefix. Rename it, and the function starts working.

   The ‘otherwise’ line is unnecessary, btw.

4. Bryan O'Sullivan says:

   2009-01-11 at 10:57

   Oops, thanks for noticing those thinkos, gentlemen. That’s what I get for composing a post at midnight!

5. Twan van Laarhoven says:

   2009-01-11 at 11:11

   Instead of writing your own ‘dropPrefix’, there is also the standard ‘stripPrefix :: Eq a => [a] -> [a] -> Maybe [a]’, which you can also use with view patterns:

   {-# LANGUAGE ViewPatterns #-}
   genbankHeader (stripPrefix “>gi|” -> Just rest) = muddle rest