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Simple arithmetic expression language
module Expressions where
import Data.Maybe
import Text.Printf
data Expr = Term Int | Op Operator Expr Expr | Var String
deriving (Show, Eq)
data Operator = Sum | Mult | Sub | Div
deriving (Show, Eq)
type Result = Either [String] Int
foldExpr :: (Int -> a) -> (Operator -> a -> a -> a) -> (String -> a) -> Expr -> a
foldExpr ft _ _ (Term i) = ft i
foldExpr ft fo fv (Op o u v) = fo o (foldExpr ft fo fv u) (foldExpr ft fo fv v)
foldExpr ft fo fv (Var id) = fv id
eval :: [(String, Int)] -> Expr -> Result
eval env = foldExpr Right applyOp getV
where
getV x = maybe (Left [x]) Right $ lookup x env
applyOp :: Operator -> Result -> Result -> Result
applyOp o (Right x) (Right y) = Right (getOp o x y)
applyOp _ (Left xs) (Left ys) = Left $ xs ++ ys
applyOp _ (Left xs) _ = Left xs
applyOp _ _ (Left ys) = Left ys
getOp :: Operator -> Int -> Int -> Int
getOp o = case o of
Sum -> (+)
Mult -> (*)
Sub -> (-)
Div -> div
printExpr :: Expr -> String
printExpr = foldExpr show (flip (printf "(%s %s %s)") . printOp) id
printOp o = case o of
Sum -> "+"
Mult -> "*"
Sub -> "-"
Div -> "/"
countTerms :: Expr -> Int
countTerms = foldExpr (const 1) (const (+)) (const 1)
sample = Op Mult (Op Sum (Var "x") (Var "y")) (Term 4)
@willtim

I had to change these lines:
26: printExpr = foldExpr show (\o u v -> printf "(%s %s %s)" u (printOp o) v)
35: countTerms = foldExpr (const 1) (_ -> (+))

@petermarks
Owner

Oops, that was slack. It seems we didn't actually compile that version in our haste at the end of the session. I've corrected it here now.

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