What is the equivalent to (+1) for the subtraction, since (-1) is seen as a negative number? [duplicate]

Answer 1

I believe you want the conveniently-named subtract function, which exists for exactly the reason you've discovered:

subtract :: Num a => a -> a -> a

the same as flip (-).

Because - is treated specially in the Haskell grammar, (- e) is not a section, but an application of prefix negation. However, (subtract exp) is equivalent to the disallowed section.

If you wanted to write it pointfree without using a function like subtract, you could use flip (-), as the Prelude documentation mentions. But that's... kinda ugly.

Answer 2

You can use the subtract function (which is in the Standard Prelude).

moveDn y = modifyMVar_ y $ return . (subtract 1)

You can also use flip to reorder the parameters that - takes.

moveDn y = modifyMVar_ y $ return . (flip (-) 1)

Answer 3

If the above-mentioned subtract is too verbose, you could try something like (+ (-1)) or (-1 +).