CODEWITHSUNDEEP
haskell
gregni
gregni

Reputation: 417

Pattern match failure : head [ ] , haskell

I give as input a list like : s = [(1,5),(1,4),(3,4)] and i want to take every time the head of this list which will be with recuircive loop 1: (1,5) loop 2: (1,4) and loop 3:(3,4). So am giving to the auxiliary function (head s) in order to take (1,5) and (tail s) in order to take [(1,4),(3,4)]

my code :

statistics :: [(Int,Int)]->(Int,Int,Int,Int,Int)
statistics [] = (0,0,0,0,0)
statistics s  =  help_statistics s (head s) (tail s) 0 0 0 0 0 0
                                                            
help_statistics :: [(Int,Int)]->(Int,Int)->[(Int,Int)]->Int->Int->Int->Int->Int->Int->(Int,Int,Int,Int,Int)
help_statistics s (x,y) taill counter matches total_points goal_for goal_against dif
                                                                            |counter==0 = if taill/=[] then help_statistics s (head taill) (tail taill) counter (matches+1) total_points goal_for goal_against dif
                                                                                        else help_statistics s (head taill) (tail taill) (counter+1) (matches) total_points goal_for goal_against dif
                                                                                        
                                                                            |otherwise  = (matches,total_points,goal_for,goal_against,dif)

Upvotes: 0

Views: 68

Answers (1)

chi
chi

Reputation: 116174

Look at this part:

if taill/=[]
then help_statistics s (head taill) ..........
else help_statistics s (head taill) .............

The first head is called on an non-empty list. The second one, on an empty list, triggering the error. You can't access the non-existent head there -- the list is empty!

You should almost never use dangerous functions like head, tail, !! in your code. They are very rarely needed in Haskell. Use instead pattern matching such as

case taill of
   (y:ys) -> use y ys        -- y is the head, ys is the tail
   []     -> something else  -- handle the empty list case somehow

In general, turn on warnings with -Wall, and always fix non-ehsaustive pattern matching is GHC warns about it by adding all the missing cases (GHC suggests those).

Upvotes: 2

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