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
Sharon Curtis and Shin-Cheng Mu have a Functional Pearl using zygomorphisms to find maximally dense segments (a generalization of maximum segment sums). Zygomorphisms are seemingly a good fit for sliding window problems once you are accustomed to them. http://www.iis.sinica.edu.tw/~scm/2010/functional-pearl-maximally-dense-segments/ I’d nominate the authors for extra credit as they’ve avoided the use of the fixed-point Mu … Read more
In the 2009-2012 period, the following things have happened: 2012: From 2012, the parallel Haskell status updates began appearing in the Parallel Haskell Digest. 2011: Parallel and Concurrent Programming in Haskell, a tutorial. version 1.1 released by Simon Marlow Haskell and parallelism, mentioned in an article in the Economist magazine, Jun 2nd 2011. Parallel tree … Read more
Both newtype and the single-constructor data introduce a single value constructor, but the value constructor introduced by newtype is strict and the value constructor introduced by data is lazy. So if you have data D = D Int newtype N = N Int Then N undefined is equivalent to undefined and causes an error when … 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
Suppose a function has side effects. If we take all the effects it produces as the input and output parameters, then the function is pure to the outside world. So, for an impure function f’ :: Int -> Int we add the RealWorld to the consideration f :: Int -> RealWorld -> (Int, RealWorld) — … Read more
GHCi now has a multiline-input mode, enabled with :set +m. For example, Prelude> :set +m Prelude> let fac 0 = 1 Prelude| fac n = n * fac (n-1) Prelude| Prelude> fac 10 3628800