Write You a Haskell Stephen Diehl

process :: String -> IO ()
process line = do
let res = parseExpr line
case res of
Left err -> print err
Right ex -> print $ runEval ex
main :: IO ()
main = runInputT defaultSettings loop
where
loop = do
minput ”
case minput of
Nothing -> outputStrLn ”Goodbye.”
Just input -> (liftIO $ process input) >> loop
Soundness
Great, now let’s test our little interpreter and indeed we see that it behaves as expected.
Arith> succ 0
succ 0
Arith> succ (succ 0)
succ (succ 0)
Arith> iszero 0
true
Arith> if false then true else false
false
Arith> iszero (pred (succ (succ 0)))
false
Arith> pred (succ 0)
0
Arith> iszero false
Cannot evaluate
Arith> if 0 then true else false
Cannot evaluate
Oh no, our calculator language allows us to evaluate terms which are syntactically valid but semantically
meaningless. We’d like to restrict the existence of such terms since when we start compiling our languages
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