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Patterns are Problematic Example
f#

I'm reading through the following paper

[link:research.microsoft.com]

and have worked through the first example

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#light

type expr =
    | Add of expr * expr
    | Mul of expr * expr
    | Var of string
    | Const of float

let e = Add(Var "x", Const 1.0)

let lookup v =
    match v with
    | "x" -> 25.0
    | "y" -> 12.0
    | _ -> 0.0

let rec eval e =
    match e with 
    | Add (e1,e2) -> eval e1 + eval e2
    | Mul (e1,e2) -> eval e1 * eval e2
    | Var x -> lookup x
    | Const n -> n

(*
let rec eval env e =
    match e with 
    | Add (e1,e2) -> eval env e1 + eval env e2
    | Mul (e1,e2) -> eval env e1 * eval env e2
    | Var x -> lookup x
    | Const n -> n
*)

Notice how i've commented out the actual code used in the example and used an eval function without the env parameter. What is the purpose of the env parameter? Is it an envelope of some kind? Is it typical to use this extra parameter in domain specific languages?

.

I haven't read the paper, but typically in such an example (any compiler/interpreter) "env" is the "environment", the extra state that's floating around that, for instance, keeps track of variable values (and so would probably be passed to the "lookup" function to translate variable names into values, rather than the 'hardcoded' example so far that defines just x and y).

By on 10/20/2008 9:12 PM ()

Ah! Of course, i feel a bit like Dr Watson [:)]

By on 10/21/2008 4:04 PM ()

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