1: The problem in the example f :: State s a f = State $ \x -> y where y = … x … is the parameter x. Things in the where clause can refer only to the parameters of the function f (there are none) and things in outer scopes. 2: To use a … Read more
Dependently Typed Haskell, Now? Haskell is, to a small extent, a dependently typed language. There is a notion of type-level data, now more sensibly typed thanks to DataKinds, and there is some means (GADTs) to give a run-time representation to type-level data. Hence, values of run-time stuff effectively show up in types, which is what … Read more
Full disclosure: I’m one of the lead developers of Snap. First of all, let’s talk about what Snap is. Right now the Snap team maintains five different projects on hackage: snap-core, snap-server, heist, snap, and xmlhtml. snap-server is a web server that exposes the API defined by snap-core. heist is a templating system. xmlhtml is … Read more
Prelude> putStrLn “\3619\3657\3634\3609\3648\3592\3657\3648\3621\3657\3591” ร้านเจ้เล้ง Note that you don’t actually have the string “\3619\3657\3634\3609\3648\3592\3657\3648\3621\3657\3591” – rather, you have the UTF-32 string ร้านเจ้เล้ง, for which “\3619\3657…” happens to be a ASCII-compliant literal. By default, GHCi uses the Show instance to display results, which doesn’t so much show things as spit out literals that can be used as … Read more
It’s a frequently asked question: How do I extract ‘the’ value from my monad, not only in Haskell, but in other languages as well. I have a theory about why this question keeps popping up, so I’ll try to answer according to that; I hope it helps. Containers of single values You can think of … Read more
I want to show you that your idea is an instance of a more general concept – zipping. Here’s a version of your program that employs a simpler and more functional style. Applicative Functors Here’s the definition of Applicative: class Functor f => Applicative f where pure :: a -> f a (<*>) :: f … Read more
Output The applicative equivalent for >> is *>, so you can do ghci> :m Control.Applicative ghci> print 5 *> print 7 5 7 Input – a better case for Applicative import Control.Applicative data Company = Company {name :: String, size :: Int} deriving Show getCompany :: IO Company getCompany = Company <$> getLine <*> readLn … Read more
My first ugly approach was: allTerms :: Omega T allTerms = do which <- each [ 1,2,3 ] if which == 1 then return A else if which == 2 then do x <- allTerms return $ B x else do x <- allTerms y <- allTerms return $ C x y But then, after … Read more
The monoid that’s referred to with “monoidal functor” is not a Monoid monoid, i.e. a value-level monoid. It’s a type-level monoid instead. Namely, the boring product monoid type Mempty = () type a <> b = (a,b) (You may notice that this is not strictly speaking a monoid; it’s only if you consider ((a,b),c) and … Read more
zip2 xs ys = foldr step done xs ys where done ys = [] step x zipsfn [] = [] step x zipsfn (y:ys) = (x, y) : (zipsfn ys) How this works: (foldr step done xs) returns a function that consumes ys; so we go down the xs list building up a nested composition … Read more