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One of the most challenging things about Haskell tutorials is the difficulty in determining the etymology of very terse function names. For example, what does

One of the most challenging things about Haskell tutorials is the difficulty in determining the etymology of very terse function names. For example, what does the M in forM_ stand for? Monad? Something else? What about liftM? Why is it called lifting? Is the M here the same as the M in forM_? These are just a few examples from this tutorial, but I find that this is pretty much common everywhere in the Haskell world, making it hard to parse semantic meaning from function names if you aren't already immersed in the language and community and well versed in the lingo. One of the best things I've learned to break out of this is to search for JavaScript implementations of many Haskell concepts when such a concept is doable in JavaScript. It usually leads me to enough understanding of what some function is trying to accomplish to proceed with whatever Haskell tutorial I happen to be reading.

dons 13y ago
Yes, the M is for monad. Eg map vs mapM
hiker 13y ago
Let me hoogle those for you (sic) http://www.haskell.org/hoogle/?hoogle=liftM http://www.haskell.org/hoogle/?hoogle=liftM =-)
dllthomas 13y ago
It's called lifting because it's moving something between levels. In the case of liftM, it's moving a function that operates on type a and returns type b, to one that operates on a wrapped type a and returns a wrapped type b. Since a monad should be a functor, and the types are the same, I'm pretty confident that this is the same thing as fmap - though in principle there could be a monad with no functor instance defined where you would have to use liftM (for now: http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal http://www.haskell.org/haskellwiki/Functor-Applicative-Monad...).
ufo 13y ago
The positive news is that while it might take a while to learn the conventions, usually the conventions are enforced all around. In your case, the conventions are documented in the docs for Control.Monad, the library those functions are from: The functions in this library use the following naming conventions: > A postfix 'M' always stands for a function in the Kleisli category: The monad type constructor m is added to function results (modulo currying) and nowhere else. So, for example, filter :: (a -> Bool) -> [a] -> [a] filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] > A postfix '_' changes the result type from (m a) to (m ()). Thus, for example: sequence :: Monad m => [m a] -> m [a] sequence_ :: Monad m => [m a] -> m () > A prefix 'm' generalizes an existing function to a monadic form. Thus, for example: sum :: Num a => [a] -> a msum :: MonadPlus m => [m a] -> m a http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html http://www.haskell.org/ghc/docs/latest/html/libraries/base/C... Personally, the things that I found hard were idioms (like learning that $ is just avoiding parenthesis, "go" is for loops, etc) and how theres a lot of libraries that make the code shorter at the expense of forcing you to be aware of them (for example, applicatives <$> <*> look alien until you learn what they are and monad transformers add a lot of "magic" to the code as well as force you to use annoying lifts all over the place)

dons 13y ago

Yes, the M is for monad. Eg map vs mapM

hiker 13y ago

Let me hoogle those for you (sic) http://www.haskell.org/hoogle/?hoogle=liftM http://www.haskell.org/hoogle/?hoogle=liftM =-)

dllthomas 13y ago

It's called lifting because it's moving something between levels. In the case of liftM, it's moving a function that operates on type a and returns type b, to one that operates on a wrapped type a and returns a wrapped type b. Since a monad should be a functor, and the types are the same, I'm pretty confident that this is the same thing as fmap - though in principle there could be a monad with no functor instance defined where you would have to use liftM (for now: http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal http://www.haskell.org/haskellwiki/Functor-Applicative-Monad...).

ufo 13y ago

The positive news is that while it might take a while to learn the conventions, usually the conventions are enforced all around. In your case, the conventions are documented in the docs for Control.Monad, the library those functions are from: The functions in this library use the following naming conventions: > A postfix 'M' always stands for a function in the Kleisli category: The monad type constructor m is added to function results (modulo currying) and nowhere else. So, for example, filter :: (a -> Bool) -> [a] -> [a] filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] > A postfix '_' changes the result type from (m a) to (m ()). Thus, for example: sequence :: Monad m => [m a] -> m [a] sequence_ :: Monad m => [m a] -> m () > A prefix 'm' generalizes an existing function to a monadic form. Thus, for example: sum :: Num a => [a] -> a msum :: MonadPlus m => [m a] -> m a http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html http://www.haskell.org/ghc/docs/latest/html/libraries/base/C... Personally, the things that I found hard were idioms (like learning that $ is just avoiding parenthesis, "go" is for loops, etc) and how theres a lot of libraries that make the code shorter at the expense of forcing you to be aware of them (for example, applicatives <$> <*> look alien until you learn what they are and monad transformers add a lot of "magic" to the code as well as force you to use annoying lifts all over the place)