CODEWITHSUNDEEP
haskellwarningsmonads
877
877

Reputation: 187

Haskell - error by execution

I try to write a this monad

data W x = W x [String]

instance Monad W where
return x = W x []
W a h1 >>= f = case f a of 
    W b h2 -> W b (h1++h2)

But, now when i will use this monad and try to write return or >>= in code I get by compilation the warnings:

No explicit method nor default method for Prelude.return in the instance declaration. No explicit method nor default method for Prelude.>>= in the instance declaration.

Does anyone know how to fix this warnings?

thank you very much

Upvotes: 2

Views: 279

Answers (1)

mokus
mokus

Reputation: 3510

Assuming that the layout of the code is exactly as displayed in your question, the problem is that your return and >>= definitions are not indented, so they are being defined as new top-level functions unrelated to the Monad class. Indent them and it should work.

Upvotes: 6

Related Questions