CODEWITHSUNDEEP
haskell
Icen
Icen

Reputation: 133

can i use a string as a whole string after cutting it to head and tails (a:as) and using recursion on it in Haskell?

I am trying to use every character in the string in a function i have (that uses only one Char) but i am also trying to use that same string as a whole in the same recursive function to compare it to indvidual characters in another string (using elem). Is there a way i can use that string heads and tails and also the whole string, so that the string will not be cut after every recursion?

Code:

checkTrue :: TrueChar -> Char -> [Char] -> TruthValue
checkTrue a b c
            | a == IsTrue b                             = AbsoluteTrue
            | (a == IsFalse b) && (b `elem` c)          = PartialTrue
            | otherwise                                 = NoneTrue

checkTruths :: [TrueChar] -> [Char] -> [TruthValue]
checkTruths [][] = []
checkTruths (a:as) (b:bs) = checkTrue a b (removeAbsoluteTrue (a:as) (b:bs)): checkTruths as bs 
{-  This is the line, 
i wanted to use b as a string and also as b:bs. is this possible? -}
checkTruths _ _ = [NoneTrue]

Upvotes: 2

Views: 67

Answers (1)

chepner
chepner

Reputation: 531718

You want an as-pattern, as documented in Section 3.17.1 of the Haskell 2010 report.

Patterns of the form var@pat are called as-patterns, and allow one to use var as a name for the value being matched by pat. For example,

case e of { xs@(x:rest) -> if x==0 then rest else xs }

is equivalent to:

let { xs = e } in
  case xs of { (x:rest) -> if x==0 then rest else xs }

In your function, you'd write

checkTruths alla@(a:as) allb@(b:bs) = checkTrue a b (removeAbsoluteTrue alla allb): checkTruths as bs

Upvotes: 5

Related Questions