im relatively new to Haskell (third day learning the language) and am having a problem with pattern matching. I have defined the function doubleEveryOther below and as far as I can see I have covered the three possible scenarios: empty list, list of length == 1 and list length > 1. The code compiles fine but when a try to use the function it throw a non-exhaustive pattern match error:
*** Exception: ex2.hs:(3,1)-(5,55): Non-exhaustive patterns in function doubleEveryOther
I have then enabled warnings in GHCI, and found the following warning when I load the ex2.hs file:
ex2.hs:3:1: Warning:
Pattern match(es) are non-exhaustive
In an equation for `doubleEveryOther':
Patterns not matched: _ : (_ : (_ : _))
Line 3:1 refers to the empty case which I think I have covered with doubleEveryOther [] = []
I cant see where I have gone wrong here. Help appreciated.
Cheers,
-- file: ex2.hs
doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther [] = []
doubleEveryOther (x:[]) = [x]
doubleEveryOther (_:[xs]) = take (length [xs] - 1) [xs]
The problem is in the third pattern:
doubleEveryOther (_:[xs])
This pattern matches the case of a list with two elements (since x:[xs] is equivalent to [x,xs]). The correct syntax is:
doubleEveryOther (_:xs)
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