(**) Syntax checker
In a certain programming language (Ada) identifiers are defined by the syntax diagram below.
Transform the syntax diagram into a system of syntax diagrams which do not contain loops; i.e. which are purely recursive. Using these modified diagrams, write a predicate identifier/1 that can check whether or not a given string is a legal identifier.
import Data.Char syntax_check :: String -> Bool syntax_check  = False syntax_check (x:xs) = isLetter x && loop xs where loop  = True loop (y:ys) | y == '-' = (not . null) ys && isAlphaNum (head ys) && loop (tail ys) | isAlphaNum y = loop ys | otherwise = False
Simple functional transcription of the diagram.
Another direct transcription of the diagram:
identifier :: String -> Bool identifier (c:cs) = isLetter c && hyphen cs where hyphen  = True hyphen ('-':cs) = alphas cs hyphen cs = alphas cs alphas  = False alphas (c:cs) = isAlphaNum c && hyphen cs
The functions hyphen and alphas correspond to states in the automaton at the start of the loop and before a compulsory alphanumeric, respectively.
Here is a solution that parses the identifier using Parsec, a parser library that is commonly used in Haskell code:
identifier x = either (const False) (const True) $ parse parser "" x where parser = letter >> many (optional (char '-') >> alphaNum)
Or we can use regular expression ( in this case Text.RegexPR ):
import Text.RegexPR import Data.Maybe identifier = isJust . matchRegexPR "^[a-zA-Z](-?[a-zA-Z0-9])*$"