QuasiExpr.hs

{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE TemplateHaskell #-}

-- Used by QuasiQuote. Example taken from the GHC documentation.
module QuasiExpr where

import Language.Haskell.TH
import Language.Haskell.TH.Quote

data Expr  =  IntExpr Integer
           |  AntiIntExpr String
           |  BinopExpr BinOp Expr Expr
           |  AntiExpr String
    deriving Show

data BinOp  =  AddOp
            |  SubOp
            |  MulOp
            |  DivOp
    deriving Show

eval :: Expr -> Integer
eval (IntExpr n)        = n
eval (BinopExpr op x y) = (opToFun op) (eval x) (eval y)
  where
    opToFun AddOp = (+)
    opToFun SubOp = (-)
    opToFun MulOp = (*)
    opToFun DivOp = div

expr = QuasiQuoter parseExprExp undefined undefined undefined

-- cheating...
parseExprExp :: String -> Q Exp
parseExprExp _ = [| BinopExpr AddOp (IntExpr 1) (IntExpr 2) |]

QuasiQuote.hs

{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE TemplateHaskell, QuasiQuotes #-}

-- example taken from the GHC documentation
module QuasiQuote where

import QuasiExpr

val :: Integer
val = eval [expr|1 + 2|]