CODEWITHSUNDEEP
haskelldesign-patterns
pioupiou1211
pioupiou1211

Reputation: 373

Error: Non-exhaustive patterns in Haskell

I have some code that deals with a list of lists (that represents a matrix). Each cell is an N or a B. I want to represent the matrix with the number of Ns for each line. For example: [[B,B,N],[N,N,N]] is represented by [1,3].

Here's my code:

data Case = B|N deriving (Show, Eq)
type Grille =[[Case]]

g0,g1 :: Grille
g0 = [[B,B,B,B,B,B,N],[B,B,N,B,N,B,B],[N,B,N,N,N,N,N],[B,B,B,N,N,B,N],[N,N,N,B,B,B,B],[B,B,B,N,N,B,N]]
g1 = [[B,B,B,B,B,B,B],[B,B,N,N,N,B,B],[B,B,N,N,N,N,N],[B,B,B,N,N,N,N],[N,N,N,B,B,B,B],[B,B,B,N,B,B,B]]

projH :: Grille -> [Int]
projH [[]] = []
projH (x:xg) = testCase(x) ++ projH(xg)

testCase :: [Case] -> [Int]dans une ligne
testCase [] = [0]
testCase (x:xs) 
    | x == N = [head(testCase(xs))+1]
    | otherwise = testCase(xs)

The problem is in the function projH, I'm obviously missing a pattern, but I do handle the case with [[],...] with the testCase function and the [[]] case. I don't know why it throws me this error: 

*Main> projH g0
[1,2,6,3,3,3*** Exception: devoir.hs:(17,1)-(18,39): Non-exhaustive patterns in function projH

Upvotes: 0

Views: 120

Answers (1)

Alistra
Alistra

Reputation: 5195

projH [] = []

instead of your 

projH [[]] = []

As the case you wrote is already handled by the other statement by assigning x = [] and xg = []

Upvotes: 4

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