Haskell - Parse error on input '->'

Question

I created this program to return a list of positions of a matrix that no has zero value.

This Code:

type Pos = (Int,Int)
type Matrix = [[Int]]

v0 [Pos]->Matrix->[Pos]
v0 [] m =[]
v0 [p:ps] m = if ((takeH m p) == 0) then v0 ps m
                                    else p:v0 ps m

takeH:: Matrix->Pos->Int
takeH m (i,j)= (m!!(i-1))!!(j-1)

Produces this error:

Parse error on input '->'
Failed,modules loades: nome.

Why ?

I hope that I've been clear.

Did you forget :: in v0 [Pos]->Matrix->[Pos] ? v0 :: [Pos]->Matrix->[Pos] — Dmytro Sirenko
– Dmytro Sirenko, Commented Oct 10, 2012 at 11:08

daknøk · Accepted Answer · 2012-10-10 11:15:53Z

4

You need :: before the type of a function.

-- vv here
v0 :: [Pos] -> Matrix -> [Pos]
v0 [] _ = []
-- v    v also parenthesis, not square brackets
v0 (p:ps) m = if ((takeH m p) == 0) then v0 ps
                                    else p:v0 ps

edited Oct 10, 2012 at 11:15

answered Oct 10, 2012 at 11:10

daknøk

6465 silver badges11 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Ashu Goel · Accepted Answer · 2012-10-11 06:49:19Z

0

Also your takeH function needs to take a Matrix and a Pos as arguments, but you only seem to pass it a Pos...? I assume you want something like takeH x (i, j) = (x!!(i-1))!!(j-1).

In general, I highly warn against using partial functions such as !!. What if your Matrix doesn't contain enough values, and goes out of bounds? Your program will terminate and throw an error. Take that into consideration as well

answered Oct 11, 2012 at 6:49

Ashu Goel

3432 gold badges4 silver badges7 bronze badges

Collectives™ on Stack Overflow

Haskell - Parse error on input '->'

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related