Let's find our way towards arrows from the perspective of monads. Consider the following IO code:
import System.IO main = do main = liftM (length . words) (readFile "jabberwocky.txt" ) >>= print -- regular functions: length, words -- Monadic functions: readFile, print
liftM to lift the composed function
length . words into the monadic function
readFile and then feed the result to another monadic function,
We can compose the regular functions with
(.), but we know well that we cannot do the following:
print . length . words . readFile "jabberwocky.txt" -- INVALID - types don't align
Let's make the preceding code possible!
The following code is based on a combination of Programming with Arrows by John Hughes, and a blog post ...