Extract a from [a]

how can 开发者_开发知识库I easily take the following

[4]

and return the following:

4

I know that [4]!!0 works but doesn't seem to be a good strategy...

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Just pattern match it:

getSingleton [a] = a

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head is the normal answer, which you see three of (one with a custom name) - this is functionally the same as what you already know (x !! 0 ~ head x). I strongly suggest against partial functions unless you can prove (with local knowledge) that you'll never pass an empty list and result in a run-time exception.

If your function doesn't guarantee a non-empty list then use listToMaybe :: [a] -> Maybe a:

> listToMaybe [4]
Just 4
> listToMaybe [5,39,-2,6,1]
Just 5
> listToMaybe []
Nothing            -- A 'Nothing' constructor instead of an exception

Once you have the Maybe a you can pattern match on that, keep it as Maybe and use fmap or a Maybe monad, or some other method to perform further operations.

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Alternatively to gatoatigrado's solution you can also use the head function, which extracts the first element of a list, but will also work on lists with more than one element and additionally is a standard function in the Prelude. You just have to be careful not to apply it to empty lists or you will get a runtime exception.

Prelude> head [4]
4
Prelude> head []
*** Exception: Prelude.head: empty list

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If you want this first item in a list you can just do

head [4]

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[] is a monad. So you use the monad "extract" operation, <-

double x = 2*x

doubleAll xs = do x <- xs
                  return (double x)

Of course, the result of the monadic computation is returned in the monad. ;)