Oh, The Humanity!
[Part 8 of the FScheme series ]
'set', 'begin', 'define'
Here we implement chapters 10 and 11 of Bill Hails’ book. We’re about to do something hideous and horrible to the language (and to our interpreter). We’re about to add assignment. This is a travesty to a pure functional programmer, but this whole thing is a learning experience and we’re not going to shy away from exploring many paradigms (I can’t wait to get to Prolog-like logic programming later).
Destructive assignment is done with ‘set!’ (“set bang”) in Scheme. Given a symbol and an expression, it finds the symbol in the environment (potentially walking the frames) and assigns the symbol a new value. It’s the first primitive we have that actually doesn’t return any useful value. To a pure functional programmer this is a very strange function. It takes arguments and returns nothing useful! It’s called for its side effects. In our case, we’ll have it return a dummy value, just for display in the REPL.
and Set env = function
| [Symbol(s); e] – >
(lookup env s) := eval env e
Dummy(sprintf "Set %s" s) | _ -> failwith "Malformed 'set!'."
With this we can say (set! x 5) or (set! x (- 7 2)) and x will be bound to 5; replacing any previous binding. It’s not like nested ‘let’s with which you can shadow the name x with a new binding. This is destructively replacing the binding in place. It’s won’t be restored when the current scope is popped.
With assignment we are entering the imperative programming paradigm. To a “normal” C# or Java dev, this style of executing a sequence of statements for their side effects seems perfectly natural. In C#, for example, there is no special syntax to construct sequences of statements. Simple juxtaposition implies it, or I suppose you could say the simicolon is kind of the “sequencing operator”. To a pure functional programmer statements (as opposed to expressions) are the most unnatural thing in the world. In Scheme we’ll have to use an explicit sequencing operation called ‘begin’.
and Begin env = List.fold (fun _ e -> eval env e) (Dummy("Empty 'begin'"))
It takes a list of statements/expressions and evaluates them in turn. The return value of the whole expression is that of the last expression in the list. For example, (begin (* 2 3) (* 4 5)) will yield 20. What happens to the (* 2 3) ? Well, it’s evaluated but the 6 is dropped on the floor; no use in that. It’s meant to be used with side-effecting statements like ‘set!’ that are called for their effects, not their values. For example, (let ((a 1)) (begin (set! a 2) (* a 10))) yields 20.
While ‘set!’ works to update existing bindings, ‘define’ is used to update existing environment frames themselves with entirely new name/value pairs. It’s primarily used in top-level code to define a set of library functions. It’s nice also at the REPL to be able to define things and then use them rather that constructing these giant ‘let’ expressions to set everything up for the body to make sense. To allow updating whole frames in place, we had to change them to refs and patch up a few things (the extend and lookup functions, etc.). I’m skipping some details. You can check out the diffs at CodePlex if you like.
and Define (env : Environment) = function
| [Symbol(s); e] – >
let def = ref (Dummy("Dummy 'define'"))
env.Head := Map.add s def env.Head.Value
def := eval env e
Dummy(sprintf "Defined %s" s) | _ -> failwith "Malformed 'define'."
Something like ‘letrec’, it first populates the current environment frame with a dummy value, then evaluates the bound expression against this, then swaps in the result. This allows for recursive definitions like (define fac (lambda (x) (if x (* x (fac (- x 1))) 1)) .
That’s about it for this set of horrible features. With them we’ve enabled imperative programming but we’ve broken referential transparency and ruined the language. Congrats! :-)
case "(let ((a 1)) (begin (set! a 2) a))" "2" // begin and assign
case "(let* ((a 5) (dummy (set! a 10))) a)" "10" // re-assign after let
case "(begin (define fac (lambda (x) (if x (* x (fac (- x 1))) 1))) (fac 7))" "5040" // define recursive
case "(begin (define square (lambda (x) (* x x))) (square 4))" "16" // global def
case "(let ((x 4)) (begin (define y 8) (* x y))))" "32" // local def