[Haskell-beginners] Trouble Understanding How to Start

[David McBride]

It might help you to remember that you can sort of replace the types in a
function with their equivalents, and it may lead you to realizing how to do
them.

For example:

get :: Name -> Env -> Int
is equivalent to
get :: String -> (String -> Maybe Int) -> Int

So think about how you can take the string you have, put it into that
function that takes a string and gives a maybe int, then how to get the int
out of the maybe int.  You'll get it.

On Sat, Oct 18, 2014 at 11:08 AM, Keeley Abbott <kallarhynn at gmail.com>
wrote:

> 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
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>
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