Hi
clunky env var1 var1 = case lookup env var1 of Nothing -> fail Just val1 -> case lookup env var2 of Nothing -> fail Just val2 -> val1 + val2 where fail = var1 + var2
Wouldn't
clunky :: Num a => [(a, a)] -> a -> a -> a clunky env var1 var2 = case (lookup var1 env, lookup var2 env) of (Just v1, Just v2) -> v1 + v2 _ -> var1 + var2
The main advantage of pattern guards (to me at least) is that they can appear on the LHS, so failing to match can result in fall through. Everything else has to come on the RHS, meaning you have pick the RHS by this point. This is bad because it now means two entirely separate alternatives just got merged into one, reducing program readability. I would love this to be in Haskell'. In fact, if we could only pick one thing to go in Haskell', I would pick pattern guards. I would have used pattern guards about 15 times today, if they were in Haskell. Compare that to rank-2 types (never needed), MPTC (needed once ever) etc. and you can see why I want this! Thanks Neil