It’s actually just a normal data constructor that happens to be defined in the Prelude, which is the standard library that is imported automatically into every module. What Maybe is, Structurally The definition looks something like this: data Maybe a = Just a | Nothing That declaration defines a type, Maybe a, which is parameterized … Read more
We can do this very efficiently by making a structure that we can index in sub-linear time. But first, {-# LANGUAGE BangPatterns #-} import Data.Function (fix) Let’s define f, but make it use ‘open recursion’ rather than call itself directly. f :: (Int -> Int) -> Int -> Int f mf 0 = 0 f … Read more
Yes, there is: Prelude> import Data.List Prelude Data.List> intercalate ” ” [“is”,”there”,”such”,”a”,”function”,”?”] “is there such a function ?” intersperse is a bit more general: Prelude> import Data.List Prelude Data.List> concat (intersperse ” ” [“is”,”there”,”such”,”a”,”function”,”?”]) “is there such a function ?” Also, for the specific case where you want to join with a space character, there … Read more
It’s a bit silly really. Every library that comes with GHC is provided in no less than 4 flavours: static dynamic profiled GHCi The GHCi version is just the static version linked together in a single .o file. The other three versions all have their own set of interface files (.hi files) too. The profiled … Read more
Lifting is more of a design pattern than a mathematical concept (although I expect someone around here will now refute me by showing how lifts are a category or something). Typically you have some data type with a parameter. Something like data Foo a = Foo { …stuff here …} Suppose you find that a … Read more
Is stack a replacement for Cabal? Yes and No. In which cases should I use Stack instead of Cabal? What can Stack do that Cabal can’t? Stack uses the curated stackage packages by default. That being so, any dependencies are known to build together, avoiding version conflict problems (which, back when they were commonplace in … Read more
Yes, it’s just syntactic sugar, with @ read aloud as “as”. ps@(p:pt) gives you names for the list: ps the list’s head : p the list’s tail: pt Without the @, you’d have to choose between (1) or (2):(3). This syntax actually works for any constructor; if you have data Tree a = Tree a … Read more
GHC also gives an option to SPECIALIZE a type-class instance declaration. I tried this with the (expanded) code of Foo.hs, by putting the following: instance (Num r, V.Vector v r, Factored m r) => Num (VT v m r) where {-# SPECIALIZE instance ( Factored m Int => Num (VT U.Vector m Int)) #-} VT … Read more
Most of the time, you can rely on type inference to work out a signature for you. In your example, the following is sufficient: Prelude> let addTwo x y = x + y If you really want a definition with a type signature, or your definition spans over multiple lines, you can do this in … Read more
They’re not the same when the second argument is negative: 2 `mod` (-3) == -1 2 `rem` (-3) == 2