What is the difference between mapM_ and mapM in Haskell? In more general terms, the difference is that mapM_ only needs to "consume" all elements of the input, whereas mapM also needs to "re-build" the data structure, with new values That's pretty much trivial for lists: you only need to traverse along the cons-cells, updating values with those received from the action you map
dictionary - map versus mapM behavior - Stack Overflow mapM is defined as mapM f as = sequence (map f as) If f returns an IO this means that for each element in the list it constructs an IO by applying f to the element If f returns an IO this means that for each element in the list it constructs an IO by applying f to the element
Order of execution with Haskells `mapM` - Stack Overflow mapM was effectively subsumed by traverse only last year, but it has existed for much longer For instance, the Haskell Report 1 3 from 1996, years before Applicative and Traversable came into being (not even ap is there, in fact), provides the following specification for mapM :
Print map functions output list using mapM_ putStrLn mapM_ :: Monad m => (a -> m b) -> [a] -> m () With the former, it acts like the normal map over a list, but where each element has an action run with the results aggregated So for example if you wanted to read multiple files you could use contents <- mapM readFile [filename1, filename2, filename3] , and contents would be a list where each
mapM for Data. Set in Haskell - Stack Overflow It provides a more general abstraction for traversals (among many other things) While it doesn't solve the problem of having an efficient mapM for Sets, it allows to express traversals for constrained data types: import Control Lens Traversal import Data Traversable (traverse) import Data Set (Set) import qualified Data Set as Set -- forall f
Is using mapM sequence considered good practice? In general, mapM makes more sense, as it is rare that we would want to do a result-yielding map on an enormous list of data More commonly, we'll want to evaluate such a list for effects, and in that case mapM_ and sequence_, which throw away the results, are typically what are recommended
Implement mapM without using sequence - Stack Overflow I am currently trying to solve the 20 Intermediate Haskell Excercises excercises and am stuck which trying to implement mapM (which is moppy in the excercise) without the use of sequence All I can produce is a [m b] by simply applying fmap but I don't know how to continue:
How does ` [ (x !! 0, x !! 1) | x lt;- mapM (const [A, B, C] ) [1 . . . The above example needs basic understanding of monads (used by mapM), and how lists are monads Furthermore it is also quite ugly and not really idiomatic Haskell Furthermore it is also quite ugly and not really idiomatic Haskell