I have a datatype called expr as follows:
data Expr = C Float | Expr :+ Expr | Expr :- Expr | Expr :* Expr| Expr :/ Expr
What I want to do is extend the Expr datatype to allow variables and 'let' expressions
Haskell Evaluating Arithmetic Expressions and Assigning Variables
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1 Answers
You need to pass around the active bindings, in what is traditionally called "the environment":
evaluate :: Environment -> Expr -> Float
There are many ways to represent an environment.
You need two operations: looking up a binding to get a value, and extending it with a new binding.
lookup :: Environment -> String -> Float
extend :: Environment -> String -> Float -> Environment
Implementation left as an exercise.
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