A monad only supplies two functions: return :: Monad m => a -> m a (>>=) :: Monad m => m a -> (a -> m b) -> m b Both of these return something of type m a, so there is no way to combine these in any way to get a function of … Read more
The vertical bar is a syntactic separator with no meaning itself, used to introduce functional dependencies on a multi-parameter type class, so technically | means nothing whatsoever. Presumably | was chosen as a visual analogy to the same symbol’s use for pattern guards on functions. As far as the functional dependencies themselves go, just read … Read more
$ is infix “application”. It’s defined as ($) :: (a -> b) -> (a -> b) f $ x = f x — or ($) f x = f x — or ($) = id It’s useful for avoiding extra parentheses: f (g x) == f $ g x. A particularly useful location for it … Read more
It’s deprecated because it was a misfeature, and didn’t actually have any useful functionality! All it did was force a bunch of extra constraints in other locations. In particular, when pattern matching on such a type, you’d be forced to add a constraint, rather than (as one might initially hope) get access to a context, … Read more
I’ve accidently written a Haskell Functors Tutorial I’ll answer your question using examples, and I’ll put the types underneath in comments. Watch out for the pattern in the types. fmap is a generalisation of map Functors are for giving you the fmap function. fmap works like map, so let’s check out map first: map (subtract … Read more
As ever, the terminology people use is not entirely consistent. There’s a variety of inspired-by-monads-but-strictly-speaking-isn’t-quite notions. The term “indexed monad” is one of a number (including “monadish” and “parameterised monad” (Atkey’s name for them)) of terms used to characterize one such notion. (Another such notion, if you’re interested, is Katsumata’s “parametric effect monad”, indexed by … Read more
Great question! There are several key differences. Representation A newtype guarantees that your data will have exactly the same representation at runtime, as the type that you wrap. While data declares a brand new data structure at runtime. So the key point here is that the construct for the newtype is guaranteed to be erased … Read more
As Michael said, the issue is buffering. By default, output is buffered until you print a newline (or until the buffer is full if you have really long lines), so you’ll most often see this issue when trying to do same-line prompts using putStr like you’re doing. I suggest defining a small helper function like … Read more
So fundamentally, you’re constrained by the type of (/): (/) :: (Fractional a) => a -> a -> a BTW, you also want Data.List.genericLength genericLength :: (Num i) => [b] -> i So how about removing the fromIntegral for something more general: import Data.List average xs = realToFrac (sum xs) / genericLength xs which has … Read more
Here is how I pronounce them: >>= bind >> then *> then -> to a -> b: a to b <- bind (as it desugars to >>=) <$> (f)map <$ map-replace by 0 <$ f: “f map-replace by 0″ <*> ap(ply) (as it is the same as Control.Monad.ap) $ (none, just as ” ” [whitespace]) … Read more