FutureLearn Haskell 2016, Parsing Text, Expression parsers

parseExpr.hs

import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Expr
import qualified Text.ParserCombinators.Parsec.Token as P
import Text.ParserCombinators.Parsec.Language
import Control.Monad (liftM)

lexer = P.makeTokenParser emptyDef

parens = P.parens lexer
natural = P.natural lexer
reservedOp = P.reservedOp lexer

data Expr =
  MkConst Integer
  | MkOpExpr String Expr Expr
  | MkPrefixOpExpr String Expr
  deriving Show

expr_parser :: Parser Expr
expr_parser = buildExpressionParser optable term <?> "expression"

optable =
  let
    op name assoc   =
      Infix ( do {  reservedOp name;
          return (MkOpExpr name) } ) assoc
    prefix name =
      Prefix  (
        reservedOp name >>
            return (MkPrefixOpExpr name) )
  in
    [ [ op "*"  AssocLeft, op "/"  AssocLeft, op "%" AssocLeft ]
    , [ op "+"  AssocLeft, op "-"  AssocLeft ], [ prefix "-" ] ]

term = parens expr_parser
       <|> liftM MkConst natural

Example of running:

~/prog> ghci parseExpr.hs
*Main> parse expr_parser  "" "3+5-9*2"
Right (MkOpExpr "-" (MkOpExpr "+" (MkConst 3) (MkConst 5)) (MkOpExpr "*" (MkConst 9) (MkConst 2)))