Pattern match list with exactly 2 elements in Haskell

15
votes

I just started learning Haskell and I'm trying to use pattern matching to match a list that has exactly 2 elements. As an exercise, I'm trying to write a function which returns the one but last element from a list. So far I found this:

myButLast :: [a] -> a
myButLast [] = error "Cannot take one but last from empty list!"
myButLast [x] = error "Cannot take one but last from list with only one element!"
myButLast [x:y] = x
myButLast (x:xs) = myButLast xs

Now the line with myButLast [x:y] is clearly incorrect, but I don't know how to match a list that has exactly 2 elements, as that is what I'm trying to do there. I read this (http://learnyouahaskell.com/syntax-in-functions#pattern-matching) page and it helped me a lot, but I'm not completely there yet...

1
haskellpattern-matching
You may either write [x,y] or x:y:[]. They are both equivalent. The pattern [x:y] means "match a list that contains exactly one element, which is itself a list, which contains exactly two elements named x and y", which isn't what you meant. – MathematicalOrchid
@MathematicalOrchid Oh, I see. Thank you very much! – user4424299
Actually, no, that's wrong... [x:y] means x is the first element and y is the remaining sublist. (Think [x:xs].) Sorry about that... Either way, it's not what you meant. – MathematicalOrchid
Also, your last rule seems in contradiction with the requirement "exactly 2 elements". I would suggest to match what you need first ([x,_]), then match anything else with a catch-all pattern (_) as an error. – cornuz

1 Answers

24
votes
myButLast :: [a] -> a
myButLast [] = error "empty list"
myButLast [x] = error "too few elements"
myButLast [x, _] = x
myButLast (x: xs) = myButLast xs

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