ghci> getAllTextMatches (“foo” =~ “o” :: AllTextMatches [] String) [“o”, “o”] I haven’t used regexes in Haskell much (which is I think Dan Burton’s answer is more idiomatic). So the way I figured this out is I looked at your type error No instance for (RegexContext Regex [Char] [String]), and popped into ghci: ghci> :t … Read more
Nope. But you can write this: data Foo = X { name :: String, age :: Int } | A { name :: String } | B { name :: String } | C { name :: String } | D { name :: String } and then have name :: Foo -> String. You … Read more
I think you can do this in general with ScopedTypeVariables which GHC supports. This certainly compiles: {-# LANGUAGE ScopedTypeVariables #-} foo :: forall a. (a -> a) -> a -> a foo f arg = bar arg where bar :: a -> a bar a = f a Note the “forall a.”
List comprehensions (in fact, Monad comprehensions) can be desugared into do notation. do i <- [1..4] j <- [i+1..4] return (i,j) Which can be desugared as usual: [1..4] >>= \i -> [i+1..4] >>= \j -> return (i,j) It is well known that a >>= \x -> return b is the same as fmap (\x -> … Read more
Understanding higher-order functions Haskell, as a functional language, supports higher-order functions (HOFs). In mathematics HOFs are called functionals, but you don’t need any mathematics to understand them. In usual imperative programming, like in Java, functions can accept values, like integers and strings, do something with them, and return back a value of some other type. … Read more
Well, one way to look at it is that Haskell translates the return type polymorphism that you’re thinking of into parametric polymorphism, using something called the dictionary-passing translation for type classes. (Though this is not the only way to implement type classes or reason about them; it’s just the most popular.) Basically, fromInteger has this … Read more
Consider the GADT data S a where S :: Show a => S a and the execution of the code foo :: S a -> a -> String foo s x = case s of S -> show x In a dictionary-based Haskell implementation, one would expect that the value s is carrying a class … Read more
What you got here is the value polymorphism restriction of ML language family. The aim of the restriction is to settle down let-polymorphism and side effects together. For example, in the following definition: let r = ref None r cannot have a polymorphic type ‘a option ref. Otherwise: let () = r := Some 1; … Read more
Using the new ArrayBuffer access arrays, it is actually possible to retrieve the exact mantissa and exponent, by extracting them from the Uint8Array. If you need more speed, consider reusing the Float64Array. function getNumberParts(x) { var float = new Float64Array(1), bytes = new Uint8Array(float.buffer); float[0] = x; var sign = bytes[7] >> 7, exponent = … Read more
I thought I would answer my own question, and list as one possible answer, the following, which is what I’m actually doing at the moment. It is quite possible that one can do better, which is why I’m asking the question! But I thought it would make sense to make the following available to people. … Read more