What is `get` supposed to do when there is no variable equal to `x`? The
type should be `Name -> Env -> Maybe Int`, there should be some additional
argument for the default value, or there should be some hard-coded default
value.
If you were using a hard-coded default value of `-1` it might look like
this:
get :: Name -> Env -> Int
get x env = case env x of
Just n -> n
Nothing -> -1
On Sat, Oct 18, 2014 at 8:08 AM, Keeley Abbott
Hi All,
So, I am taking a Functional Programming class this fall, and I have struggling to understand how to build types and functions correctly. For some reason it just isn’t clicking. I have the following module I am working on, and I need some guidance.
— Variable names type Name = String
— An environment for looking up the value of a variable type Env = Name -> Maybe Int
— An empty environment empty :: Env empty env = Nothing
— Set a variable to a value in the environment if it doesn’t already exist set :: Name -> Int -> Env -> Env set x i e = \y -> if x == y then (Just i) else e y
— Lookup the value of a variable in the environment get :: Name -> Env -> Int get x env = ?? (I previously had “type Env = Name -> Int”, so this was working with env x, but we had to change empty to be something other than just a runtime error, which is why I made Env = Name -> Maybe Int)
—Remove a variable from the environment unset :: Name -> Env -> Env unset x e = \y -> if x == y then empty x else env y
At this point I don’t know how to fix get, so I get an int to display (it keeps telling me it can’t match the type ‘Maybe Int’ with the expected type ‘Int’). And I don’t know where to go from there to create getOr, setAll, and mapEnv functions, because I am just not understanding what I am doing. For the most part I have been doing simpler things that I could just mess with the functions until they work, but even at that I’m not getting HOW they work… Any assistance or instruction on how I can GET what I am trying to do would be greatly appreciated.
Thanks, Kallarhynn _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners