Haskell: use list generated in IO function in another, non-IO function

Question

I am using a String -> IO [x] function in order to read in the contents of a file into a list where each element in the list is a word from the file.

However, I would like to use the the [x] list as an input for another function. My issue is that I do not understand how to access the basic [x] list rather than the IO [x] list generated in my initial function. I have been getting around this for testing by using GHCI and using

k <- listRead "file.txt"

and then using that k as input in my next function that only takes a list [x] as input.

How can I use the "internal" list [x] instead of the returned IO [x] that my initial function returns?

You can't since an `IO a` is not the *result* of an IO operation, it is a "recipe" for an IO operation. You can however use the `k` variable, and thus make a function call. — Willem Van Onsem, Mar 17 '20 at 22:08

chepner · Answer 1 · 2020-03-17T22:44:00.083

3

This is what the Monad type class is for.

foo :: String -> IO [SomeType]
bar :: [SomeType] -> IO SomeOtherType

then

baz :: String -> IO SomeOtherType
baz name = foo name >>= bar

(or after importing Control.Monad, baz = foo >=> bar).

If all you have is some function bar' :: [SomeType] -> SomeOtherType, then you can define

bar :: [SomeType] -> IO SomeOtherType
bar = return . bar'

Note you can never actually "get" the value of type [SomeType]; you can only create new IO actions that use the value once the IO action is executed.

edited Mar 17 '20 at 22:44

answered Mar 17 '20 at 22:17

chepner

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An alternative to `bar = return . bar'` would be to use the `Functor` instance for `IO`, giving `baz = fmap bar' . foo`. – Khuldraeseth na'Barya Mar 17 '20 at 23:31

Will Ness · Answer 2 · 2020-03-22T20:15:12.560

Simply, put that line which you used in GHCi into a do block:

do
    k <- listRead "file.txt"    
    let r = yourPureFunction k
    return r

In do notation, for an x <- mx line, whenever mx :: M a for some Monad M, we have x :: a.

Thus yourPureFunction :: a -> b is applied to the a argument on the inside of M, and the combined monadic value represented by the do block has the type M b, because return has the type return :: b -> M b, and the overall type of a do block is the type of its last expression.

The above code happens to really be a higher-level syntax representation of / encodable as / (return . yourPureFunction) =<< listRead "file.txt" which is also the same as yourPureFunction <$> listRead "file.txt" (with (<$>) = fmap), but that's besides the point.

Haskell: use list generated in IO function in another, non-IO function

2 Answers2