The problem is twofold. First of all, Haskell is indentation sensitive, e.g. the following code isn’t valid: example = (a, b) where a = “Hello” b = “World” Both bindings need to be indented with the same number of spaces/tabs (see off-side rule). While it’s obvious in this case, it’s rather hidden in the following … Read more
Whenever possible, reuse library code. import Data.Map sortAndGroup assocs = fromListWith (++) [(k, [v]) | (k, v) <- assocs] Try it out in ghci: *Main> sortAndGroup [(1,”aa”),(1,”cc”),(2,”aa”),(3,”ff”),(3,”gg”),(1,”bb”)] fromList [(1,[“bb”,”cc”,”aa”]),(2,[“aa”]),(3,[“gg”,”ff”])] EDIT In the comments, some folks are worried about whether (++) or flip (++) is the right choice. The documentation doesn’t say which way things get … Read more
I’m going to order this guide by the level of skill you have in Haskell, going from an absolute beginner right up to an expert. Note that this process will take many months (years?), so it is rather long. Absolute Beginner Firstly, Haskell is capable of anything, with enough skill. It is very fast (behind … Read more
First: The term monad is a bit vacuous if you are not a mathematician. An alternative term is computation builder which is a bit more descriptive of what they are actually useful for. They are a pattern for chaining operations. It looks a bit like method chaining in object-oriented languages, but the mechanism is slightly … Read more
Try turning on warnings in ghci. This enables the compile time warnings you can get with ghc by passing -W, for example. You can do this several ways: ghci -fwarn-incomplete-patterns Or Neil Mitchell describes how he sets this up in his .ghci. Here is the relevant excerpt: :set -fwarn-incomplete-patterns You can manually enter this at … Read more
The $ operator is for avoiding parentheses. Anything appearing after it will take precedence over anything that comes before. For example, let’s say you’ve got a line that reads: putStrLn (show (1 + 1)) If you want to get rid of those parentheses, any of the following lines would also do the same thing: putStrLn … Read more
What is the monomorphism restriction? The monomorphism restriction as stated by the Haskell wiki is: a counter-intuitive rule in Haskell type inference. If you forget to provide a type signature, sometimes this rule will fill the free type variables with specific types using “type defaulting” rules. What this means is that, in some circumstances, if … Read more
The cause for the error you’re getting here is the use of the name count&remove: identifiers in Haskell have to be either alphanumeric (e.g. count_and_remove) or symbolic (e.g. ==, ++, etc.). Identifiers can’t contain both alphanumeric characters and symbols. But that’s just one problem. You also have a reference to y on line 9 that … Read more
There are following approaches. 1) Create a list pf pairs which are all combinations [(10,10),(10,15),..,(15,10),(15,3)..]. Now you can use simple any function on this list to check if any pair add up to given number. getCoupleList :: [a]->[(a,a)] getCoupleList [] = [] getCoupleList [x] = [] getCoupleList (x:xs) = map (\y->(x,y)) xs ++ getCoupleList xs … Read more
Three things that I can see: You have the arguments that you pass to scramble in main the wrong way round. You haven’t defined getContents anywhere. You’re not using getArgs as you require. As you guessed, you only need to get the first argument (as getArgs :: IO [String]); using arg1:_ <- getArgs will indeed … Read more