I want to concatenate strings in Haskell and also integers from function like this:
arc 13 34 234 3
13 34 234 3 will be arguments of arc function and I want output like
"arc(13, 34, 234, 3)"
as a String how can I implement this?
2 Answers
How can list of numbers could be concatenated into a string? Looks like some [Int] -> String function can help here.
> concat . intersperse ", " . map show $ [13, 34, 234, 3]
"13, 34, 234, 3"
So, let's add some brackets and "arc" to that String.
import Data.List (intersperse)
arc :: [Int] -> String
arc nums = "arc(" ++ (concat . intersperse ", " . map show $ nums) ++ ")"
And we get the answer.
> arc [13, 34, 234, 3]
"arc(13, 34, 234, 3)"
If you are really need function with signature like Int -> Int -> Int -> Int -> String:
arc' :: Int -> Int -> Int -> Int -> String
arc' a1 a2 a3 a4 = "arc(" ++ (concat . intersperse ", " . map show $ [a1,a2,a3,a4]) ++ ")"
> arc' 13 34 234 3
"arc(13, 34, 234, 3)"
Comments
If you want String output, the typical technique is to create a ShowS, which is just another name for String -> String.
showsArc :: Int -> Int -> Int -> Int -> ShowS
showsArc a b c d = showString "arc" . shows (a, b, c, d)
>>> showsArc 13 34 234 3 []
"arc(13,34,234,3)"
The [] at the end of the function call is just an empty string. It lets you attach data to the end without worrying about O(N) string concatenation.
>>> showsArc 13 34 234 3 " and some extra text"
"arc(13,34,234,3) and some extra text"
2 Comments
Mahmut Bulut
i tried this but ghci gives ˙Arc.hs:30:1: Invalid type signature: Arc :: Int -> Int -> Int -> Int -> ShowS Should be of form <variable> :: <type> ˙
Dietrich Epp
You can't name a function (or value) starting with a capital letter:
Arc is an invalid function name in Haskell. (Only constructors, types, modules, and classes are named starting with capital letters.)
arcfunction should have 4Intarguments? Maybe [Int] will be more usable?