[Haskell-cafe] Is it possible to change the environment (reader) in applicative style?
Rodrigo Stevaux
roehst at gmail.com
Wed Sep 12 15:12:42 UTC 2018
Yes, the example with Let Name Term Term is what I was experimenting with.
About "eval t2 . (update' <*> pure s <*> eval t1)":
Well I was following applicative style as "Applicative Programming
with Effects" by Conor McBride
I did not consider this line applicative because of the (.) operator;
I am trying to get away with just `pure` and `<*>` -- to be more
precise, the K and S combinators.
So the question becomes: can we implement the environment modification
operation without resorting to function composition?
Em qua, 12 de set de 2018 às 05:10, Ivan Perez
<ivanperezdominguez at gmail.com> escreveu:
>
> On 11 September 2018 at 21:50, Rodrigo Stevaux <roehst at gmail.com> wrote:
>>
>> It is easy to read an environment in applicative style, ie:
>>
>> type Env = [(String, Int)]
>> data Term = Add Term Term | Number Int | Var String deriving Show
>> eval :: Term -> Env -> Int
>> eval (Add a b) = (+) <$> eval a <*> eval b
>> eval (Var name) = fetch name
>> eval (Number i) = pure i
>>
>> fetch :: String -> Env -> Int
>> fetch name = fromJust . lookup name
>>
>> But can the eval function change the Env being passed, as to implement
>> a "let" operation, without using monads? I tried I lot but ultimately
>> I resorted to (>>=) in the function monad:
>>
>> bind f k = \r -> k (f r) r
>
>
> I think what you mean is something like: can we extend Term with a let binding expression and implement eval using applicative interface without (>>=)?
>
> I think we can, and it's a bit awkward, but possible, because of the Reader monad.
>
> A trivial way of introducing let that does not manifest the issues you point out is
>
> data Term = Add Term Term | Number Int | Var String | Let String Int Term
>
> You can then implement the case for eval with
>
> eval (Let s v t) = eval t . update s v
>
> where the function update simply updates a value in the associative list. A simple implementation is:
>
> update :: Ord a => a -> b -> [(a, b)] -> [(a, b)]
> update s v = nubBy eqFst . insertBy cmpFst (s, v)
> where
> eqFst x y = (==) (fst x) (fst y)
> cmpFst x y = compare (fst x) (fst y)
>
> Of course, this does not need the monad interface, but it does not really need the applicative interface to evaluate the term either (except indirectly in eval t).
>
> Perhaps a more interesting alternative is:
>
> data Term = ... | LetT String Term Term
>
> where the other cases in Term remain the same. Now you need to eval the first term to change the environment, which is, I guess, what you wanted?
>
> You can do this combining composition with applicative:
>
> eval (LetT s t1 t2) = eval t2 . (update' <*> pure s <*> eval t1)
> where
> update' :: Env -> String -> Int -> Env
> update' e s v = update s v e
>
> And a test (which is equivalent to let b = a + 8 in b + 1):
>
> *Main> eval (LetT "b" (Add (Number 8) (Var "a")) (Add (Number 1) (Var "b"))) [("a", 7)]
> 16
>
>
>>
>>
>> I do not think so, because in applicative style each operand can have
>> an effect (reading the environment) but can not affect other operands
>> (including the next ones), i.e., there is no notion of sequencing in
>> applicatives
>> Is this reasoning right?
>
>
> As Tom pointed out, not 100%, not generally, I think. This seems to be specific to the reader monad.
>
> All the best,
>
> Ivan
>
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