On Tue, Sep 11, 2018 at 10:50:41PM -0300, Rodrigo Stevaux 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'm confused by this. Your `bind` doesn't seem to change the Env being passed. Can you explain?
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?
Your conclusion may be right but I don't think that your reasoning is. Certainly an applicative action can affect the subsequent operations. State is Applicative after all! Tom