eval for mini language.roc

gistfile1.txt

Expr : [
    Func Str Expr,
    Call Expr Expr, 
    Symbol Str, 
    Add Expr Expr, 
    Const I32, 
]

eval: Expr, Dict Str Expr -> Result Expr [ExpectedFunction, SymbolNotFound Str, ExpectedI32]
eval = \ast, scope ->
    when ast is 
        Func _ _ as f -> Ok f
        Call fn arg ->
            when eval? fn scope is
                Func name expr -> 
                    eval expr (Dict.insert scope name arg)
                _ -> Err ExpectedFunction
        Symbol name ->  
            when Dict.get scope name is
                Ok lookup -> eval lookup scope
                _ -> Err (SymbolNotFound name)
        Add left right ->
            when (eval? left scope, eval? right scope) is
                (Const lhs, Const rhs) -> Ok (Const (lhs + rhs))
                _ -> Err ExpectedI32
        Const v -> Ok (Const v)

(Add (Const 5) (Const 5))
|> eval (Dict.empty {})