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(**) 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])*$"