问题
Possible Duplicate:
Currying subtraction
I started my first haskell project that is not from a tutorial, and of course I stumble on the simplest things.
I have the following code:
moveUp y = modifyMVar_ y $ return . (+1)
moveDn y = modifyMVar_ y $ return . (-1)
It took me some time to understand why my code wouldn't compile: I had used (-1) which is seen as negative one. Bracketting the minus doesn't help as it prefixes it and makes 1 its first parameter.
In short, what is the point free version of this?
dec :: Num a => a -> a
dec x = x - 1
回答1:
I believe you want the conveniently-named subtract function, which exists for exactly the reason you've discovered:
subtract :: Num a => a -> a -> athe 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.
回答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)
回答3:
If the above-mentioned subtract is too verbose, you could try something like (+ (-1)) or (-1 +).
来源：https://stackoverflow.com/questions/12831217/what-is-the-equivalent-to-1-for-the-subtraction-since-1-is-seen-as-a-nega