CODEWITHSUNDEEP
haskellexceptionpattern-matching
Alfred Kreuzer
Alfred Kreuzer

Reputation: 23

Haskell pattern matching - Non-exhaustive pattern - but why?

I'm trying to create a run-length encoding function in Haskell. I know there are lot's of examples out there, I'm not really looking for a solution but more for an explanation why mine doesn't work.

rle [] s acc = acc
rle (x:xs) s acc = rle xs s ((x, count x s) : acc)

count x = length . filter (x==)

For some reason Haskell isn't able to pattern match this. e.g. rle "abcd" "aaa" [] should return [('a',3),('b',0),...] but always causes a Non-exhaustive patterns Exception.

Am I missing something obvious here?

Upvotes: 1

Views: 117

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476624

I assume that you defined this in ghci. If you write functions that span multiple lines, you should surround these by :{ and :}. If you do not do that, you define two functions rle. The second one will be more locally scoped, hence if you later call rle, it will pick that one.

For example:

$ ghci
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/kommusoft/.ghci
Prelude> :{
Prelude| rle [] s acc = acc
Prelude| rle (x:xs) s acc = rle xs s ((x, count x s) : acc)
Prelude| 
Prelude| count x = length . filter (x==)
Prelude| :}
Prelude> rle "abcd" "aaa" []
[('d',0),('c',0),('b',0),('a',3)]

Working with count will however not work here, since your rle each time makes one step in the list. Furthermore you here do not need to use an accumulator.

Upvotes: 2

Related Questions