It isn’t really practical to have a doubly linked list in Haskell, because you have to construct it all at once. For example, imagine that you have a list [1, 2, 3, 4, 5] that you want to make doubly linked. Now, let’s imagine how the list is represented: data DoubleList a = LeftEnd a … Read more
hammar’s already provided a solution; I’d like to point out another problem with this example. What you want to express is “Whenever a type is an instance of Eq and EuclideanDomain, use this rule to make an instance for Ord.” But this is inexpressible in Haskell. The line instance (EuclideanDomain a, Eq a) => Ord … Read more
This is a common barrier for new Haskell programmers. You want to escape IO, and it takes a while to figure out the best way to do so. The Learn You A Haskell tutorial has a good explanation of one way to generate random numbers using the state monad, but it still has to be … Read more
It’s probably best to look at what SafeHaskell allows: Safe Language The Safe Language (enabled through -XSafe) restricts things in two different ways: Certain GHC LANGUAGE extensions are disallowed completely. Certain GHC LANGUAGE extensions are restricted in functionality. Below is precisely what flags and extensions fall into each category: Disallowed completely: GeneralizedNewtypeDeriving, TemplateHaskell Restricted functionality: … Read more
First of all, remember how fmap is defined for applicatives: fmap f x = pure f <*> x This means that your example is the same as (fmap (+ 5) (* 3)) 4. The fmap function for functions is just composition, so your exact expression is the same as ((+ 5) . (* 3)) 4. … Read more
I think the best explanation I’ve heard was actually fairly recently on SO. IO Foo is a recipe for creating a Foo. Another common, more literal, way of saying this is that it is a “program that produces a Foo“. It can be executed (many times) to create a Foo or die trying. The execution … Read more
Here’s a fully parametric example: data Weird a = Weird a (a -> a) instance Foldable Weird where foldMap f (Weird a b) = f $ b a Weird is not a Functor because a occurs in a negative position.
A lens consists of two functions, a getter and a setter: data Lens a b = Lens { getter :: a -> b, setter :: b -> a -> a } For example, we might have lenses for the first and second parts of a pair: fstLens :: Lens (a, b) a fstLens = Lens … Read more
The s keeps objects inside the ST monad from leaking to the outside of the ST monad. — This is an error… but let’s pretend for a moment… let a = runST $ newSTRef (15 :: Int) b = runST $ writeSTRef a 20 c = runST $ readSTRef a in b `seq` c Okay, … Read more
Hmm! Some archaeology! Since around 2004 I’ve used go as the generic name for tail-recursive worker loops, when doing a worker/wrapper transformation of a recursive function. I started using it widely in bytestring, e.g. foldr :: (Word8 -> a -> a) -> a -> ByteString -> a foldr k v (PS x s l) = … Read more