This problem represents a turning point between object-oriented and functional thinking. Sometimes even sophisticated Haskellers are still in this mental transition, and their designs often fall into the existential typeclass pattern, mentioned in Thomas’s answer. A functional solution to this problem involves reifying the typeclass into a data type (usually once this is done, the … Read more
What are n+k patterns? Take a gander at this: $ ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ 😕 for help Loading package ghc-prim … linking … done. Loading package integer-gmp … linking … done. Loading package base … linking … done. Loading package ffi-1.0 … linking … done. Prelude> let f 0 = 0 ; f (n+5) … Read more
I’ve enabled library-profiling: True in my ~/.cabal/config file. From then on, any new installations will automatically enable profiling. Unfortunately that still means I had to manually reinstall for the old packages already installed. Although, after a while of doing this manually, I now have most packages reinstalled with profiling enabled…
This answer is in two parts. The first part addresses the question directly. The second part goes off on a tangent (literally) in order to dig about in the mathematics behind the first part: it may thus prove to be difficult material of limited interest, but I thought a few extremists might enjoy it. The … Read more
Here’s a different and simpler function that calculates the n’th Fibonacci number: fib :: Integer -> Integer fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) The implementation you are referring to relays on some observations about how values in Fibonacci relate to each other (and how Haskell … Read more
MonadPlus and Monoid serve different purposes. A Monoid is parameterized over a type of kind *. class Monoid m where mempty :: m mappend :: m -> m -> m and so it can be instantiated for almost any type for which there is an obvious operator that is associative and which has a unit. … Read more
You are doing nothing wrong. fix id is an infinite loop. When we say that fix returns the least fixed point of a function, we mean that in the domain theory sense. So fix (\x -> 2*x-1) is not going to return 1, because although 1 is a fixed point of that function, it is … Read more
Lists Rock By far the most friendly data structure for sequential data in Haskell is the List data [a] = a:[a] | [] Lists give you Ï´(1) cons and pattern matching. The standard library, and for that matter the prelude, is full of useful list functions that should litter your code (foldr,map,filter). Lists are persistant … Read more
Regarding monads: Yes, Either a is a monad. So simplifying the problem, you are basically asking for this: main = print $ magicMonadUnwrap v v :: Either String Int v = Right 3 magicMonadUnwrap :: (Monad m) => m a -> a magicMonadUnwrap = undefined How do you define magicMonadUnwrap? Well, you see, it’s different … Read more
There’s another factor here, mentioned in some of the links which acfoltzer includes, but it might be worth making explicit here. You’re encountering the effect of the monomorphism restriction. When you say let x = 5 you make a top-level definition of a variable. The MR insists that such definitions, when otherwise unaccompanied by a … Read more