Haskell parse error on input `=' on a simple line

Question

I'm trying to learn Haskell and I got this error.
parse error on input `=' Here is my code:

nAnd1 :: Bool -> Bool -> Bool
nAnd x y = if (x==False && y == False) || x/=y then True else False

nAnd2 :: Bool -> Bool -> Bool
nAnd x y | if ((x == False && y == False) || x/=y) = True
         | otherwise = False

The place where the error takes place is at the "=" before True in nAnd2. Any solution?

but why after definition of type in nAnd1 and nAnd2 you write nAnd? — SergeyKuz1001
– SergeyKuz1001, Commented Sep 25, 2020 at 9:39
if ... then True else False is just a long way of writing .... — chepner
– chepner, Commented Sep 25, 2020 at 13:19

Franky · Accepted Answer · 2020-09-25 10:58:21Z

3

The answer to drop the if was already given. I suppose the exercise was meant differently. The nAnd means "not and", i.e.

nAnd a b = not (a && b)

but you have nAnd x y = (not x && not y) || x /= y. That is very strange and I suspect an X-Y-Problem here. Did you get the definition of nand from a truth table?

  a  |  b  | a `nAnd` b
-----+-----+----------
False|False|   True
False|True |   True
True |False|   True
True |True |   False

Then take the time to rewrite the truth table as

  a  |  b  | a `nAnd` b
-----+-----+----------
True |True |   False
  *  |  *  |   True

and use Pattern Matching:

nAnd :: Bool -> Bool -> Bool
nAnd True True = False
nAnd _    _    = True

answered Sep 25, 2020 at 10:58

Franky

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willeM_ Van Onsem · Accepted Answer · 2020-09-25 10:45:41Z

2

The guards (|) act as if clauses, it does not make sense to write if for these:

nAnd :: Bool -> Bool -> Bool
nAnd x y | (x == False && y == False) || x /= y = True
         | otherwise = False

It furthermore however is odd to return True or False based on a condition, you can rewrite this to:

nAnd :: Bool -> Bool -> Bool
nAnd x y = (not x && not y) || x /= y

or we can negate the values and use (||):

import Data.Function(on)

nAnd :: Bool -> Bool -> Bool
nAnd = on (||) not

edited Sep 25, 2020 at 10:45

answered Sep 25, 2020 at 10:08

willeM_ Van Onsem

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