Haskell Parse Error in Helper Function

Question

I'm just learning Haskell, and I decided to try writing my own version of pred, which returns the number preceding its parameter. I'm using WinGHCi and loading a file called test.hs. Here is my code:

prev :: (Num a) => a -> a
prev x = prev' 0 x
  where prev' y z
    | (succ y) == z = y
    | otherwise = prev' (succ y) z

I get the error:

test.hs:4:5:
    parse error (possibly incorrect indentation or mismatched brackets)

How does one properly write a helper function with guards?

The standard answer here is to be sure you're using spaces, not tabs.. — J. Abrahamson
– J. Abrahamson, Commented Dec 31, 2013 at 7:12

Luis Casillas · Accepted Answer · 2013-12-31 02:45:57Z

2

Your pattern guards need to be more deeply indented—they need to be further to the right than the p in the definition of prev'. This version doesn't give the parse error:

prev :: (Num a) => a -> a
prev x = prev' 0 x
  where prev' y z
            | (succ y) == z = y
            | otherwise = prev' (succ y) z

You're still getting a type error there, though—need more type class constraints.

answered Dec 31, 2013 at 2:45

Luis Casillas

30.3k7 gold badges51 silver badges103 bronze badges

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

2 Comments

Numeri Over a year ago

Thanks for the great answer! What do you mean by type class constraints?

Numeri Over a year ago

I got it - I need to replace (Num a) by something like (Integral a) so it can test for equality. Thanks!

Collectives™ on Stack Overflow

Haskell Parse Error in Helper Function

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related