Question

I am trying to implement collatz-list using Haskel:
Here’s my code:

collatz n
     | mod n 2 == 0 = div n 2
     | otherwise =  3 * n + 1


collatzList n
        | n < 1 = error "Cannot have negative number"
collatzList
        | n == 1 = [n]
        | otherwise = n:collatzList (collatz n)

The error message I am getting is this:
parse error on input `collatzList’
[1 of 1] Compiling Main ( exer.hs, interpreted )
Failed, modules loaded: none.

Can anyone tell me why I am getting this message?

Answer 1

I get different errors (using GHC 7.4.1):

> :load "/tmp/C.hs"
[1 of 1] Compiling Main             ( /tmp/C.hs, interpreted )

/tmp/C.hs:9:11: Not in scope: `n'

/tmp/C.hs:9:21: Not in scope: `n'

/tmp/C.hs:10:23: Not in scope: `n'

/tmp/C.hs:10:46: Not in scope: `n'
Failed, modules loaded: none.

This is because you forgot the n argument in your second equation for collatzList. You can either add this argument

collatzList n
        | n < 1 = error "Cannot have negative number"
collatzList n -- the n was missing here
        | n == 1 = [n]
        | otherwise = n:collatzList (collatz n)

or, since the left hand sides are now the same, you can simply join it with the first one:

collatzList n
        | n < 1 = error "Cannot have negative number"
        | n == 1 = [n]
        | otherwise = n:collatzList (collatz n)