Haskell Pattern Matching Fails on Negative Number

Question

The Haskell compiler throws an error on the following function:

balancedMax :: Int -> Int -> Int
balancedMax -1 _ = -1
balancedMax _ -1 = -1
balancedMax a b = max a b

Flipping the sign solves the problem:

balancedMax :: Int -> Int -> Int
balancedMax 1 _ = -1
balancedMax _ 1 = -1
balancedMax a b = max a b

Why does pattern matching fail on the negatives, and what is a clean workaround?

Answer 1

It fails because it thinks you're trying to re-define the minus operator, because f -1 = ... gets parsed as f - 1 = ....

To fix this you just have to add parentheses:

balancedMax :: Int -> Int -> Int
balancedMax (-1) _ = -1
balancedMax _ (-1) = -1
balancedMax a b = max a b

The same thing happens in expressions. To call balancedMax with a negative literal, you would need parentheses as well.

Answer 2

because f -1 = ... gets parsed as f - 1 = ....

Which if you don't know, means the same thing as:

(-) f 1 = ....

which is similar to defining a function like this:

somefunc x 1 = x + 1

Which happens to be equivalent to defining somefunc() like this:

x `somefunc` 1 = x + 1

It's just that with (-) you don't have to write the backticks when using it in infix position, i.e. positioned between its two arguments.

The two beginning haskell books I've looked at both warn you early on that the unary - needs to be used with parentheses, e.g. (-3).