Created

Embed URL

HTTPS clone URL

SSH clone URL

You can clone with HTTPS or SSH.

Download Gist

Simple arithmetic expression language

View expressions.hs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
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)
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 -> Int
eval env = foldExpr id getOp getV
where
getV x = fromMaybe (error $ "unbound identifier: " ++ x) (lookup x env)
 
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") (Term 3)) (Term 4)

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) (_ -> (+))

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.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Something went wrong with that request. Please try again.