Lecture #9, Tuesday, February 6th
=================================

- Evaluating with Substitution Caches
- Dynamic and Lexical Scopes

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# Evaluating with Substitution Caches

Implementing the new `eval` is easy now --- it is extended in the same
way that the formal `eval` rule is extended:

    (: eval : FLANG SubstCache -> FLANG)
    ;; evaluates FLANG expressions by reducing them to expressions
    (define (eval expr sc)
      (cases expr
        [(Num n) expr]
        [(Add l r) (arith-op + (eval l sc) (eval r sc))]
        [(Sub l r) (arith-op - (eval l sc) (eval r sc))]
        [(Mul l r) (arith-op * (eval l sc) (eval r sc))]
        [(Div l r) (arith-op / (eval l sc) (eval r sc))]
        [(With bound-id named-expr bound-body)
         (eval bound-body
               (extend bound-id (eval named-expr sc) sc))]
        [(Id name) (lookup name sc)]
        [(Fun bound-id bound-body) expr]
        [(Call fun-expr arg-expr)
         (define fval (eval fun-expr sc))
         (cases fval
           [(Fun bound-id bound-body)
            (eval bound-body (extend bound-id (eval arg-expr sc) sc))]
           [else (error 'eval "`call' expects a function, got: ~s"
                        fval)])]))

Again, note that we don't need `subst` anymore, but the rest of the code
(the data type definition, parsing, and `arith-op`) is exactly the same.

Finally, we need to make sure that `eval` is initially called with an
empty cache. This is easy to change in our main `run` entry point:

    (: run : String -> Number)
    ;; evaluate a FLANG program contained in a string
    (define (run str)
      (let ([result (eval (parse str) empty-subst)])
        (cases result
          [(Num n) n]
          [else (error 'run "evaluation returned a non-number: ~s"
                       result)])))

The full code (including the same tests, but not including formal rules
for now) follows. Note that one test does not pass.

    #lang pl

    (define-type FLANG
      [Num  Number]
      [Add  FLANG FLANG]
      [Sub  FLANG FLANG]
      [Mul  FLANG FLANG]
      [Div  FLANG FLANG]
      [Id   Symbol]
      [With Symbol FLANG FLANG]
      [Fun  Symbol FLANG]
      [Call FLANG FLANG])

    (: parse-sexpr : Sexpr -> FLANG)
    ;; parses s-expressions into FLANGs
    (define (parse-sexpr sexpr)
      (match sexpr
        [(number: n)    (Num n)]
        [(symbol: name) (Id name)]
        [(cons 'with more)
         (match sexpr
           [(list 'with (list (symbol: name) named) body)
            (With name (parse-sexpr named) (parse-sexpr body))]
           [else (error 'parse-sexpr "bad `with' syntax in ~s" sexpr)])]
        [(cons 'fun more)
         (match sexpr
           [(list 'fun (list (symbol: name)) body)
            (Fun name (parse-sexpr body))]
           [else (error 'parse-sexpr "bad `fun' syntax in ~s" sexpr)])]
        [(list '+ lhs rhs) (Add (parse-sexpr lhs) (parse-sexpr rhs))]
        [(list '- lhs rhs) (Sub (parse-sexpr lhs) (parse-sexpr rhs))]
        [(list '* lhs rhs) (Mul (parse-sexpr lhs) (parse-sexpr rhs))]
        [(list '/ lhs rhs) (Div (parse-sexpr lhs) (parse-sexpr rhs))]
        [(list 'call fun arg)
                           (Call (parse-sexpr fun) (parse-sexpr arg))]
        [else (error 'parse-sexpr "bad syntax in ~s" sexpr)]))

    (: parse : String -> FLANG)
    ;; parses a string containing a FLANG expression to a FLANG AST
    (define (parse str)
      (parse-sexpr (string->sexpr str)))

    ;; a type for substitution caches:
    (define-type SubstCache = (Listof (List Symbol FLANG)))

    (: empty-subst : SubstCache)
    (define empty-subst null)

    (: extend : Symbol FLANG SubstCache -> SubstCache)
    ;; extend a given substitution cache with a new mapping
    (define (extend name val sc)
      (cons (list name val) sc))

    (: lookup : Symbol SubstCache -> FLANG)
    ;; lookup a symbol in a substitution cache, return the value it is
    ;; bound to (or throw an error if it isn't bound)
    (define (lookup name sc)
      (let ([cell (assq name sc)])
        (if cell
          (second cell)
          (error 'lookup "no binding for ~s" name))))

    (: Num->number : FLANG -> Number)
    ;; convert a FLANG number to a Racket one
    (define (Num->number e)
      (cases e
        [(Num n) n]
        [else (error 'arith-op "expected a number, got: ~s" e)]))

    (: arith-op : (Number Number -> Number) FLANG FLANG -> FLANG)
    ;; gets a Racket numeric binary operator, and uses it within a FLANG
    ;; `Num' wrapper
    (define (arith-op op val1 val2)
      (Num (op (Num->number val1) (Num->number val2))))

    (: eval : FLANG SubstCache -> FLANG)
    ;; evaluates FLANG expressions by reducing them to expressions
    (define (eval expr sc)
      (cases expr
        [(Num n) expr]
        [(Add l r) (arith-op + (eval l sc) (eval r sc))]
        [(Sub l r) (arith-op - (eval l sc) (eval r sc))]
        [(Mul l r) (arith-op * (eval l sc) (eval r sc))]
        [(Div l r) (arith-op / (eval l sc) (eval r sc))]
        [(With bound-id named-expr bound-body)
         (eval bound-body
               (extend bound-id (eval named-expr sc) sc))]
        [(Id name) (lookup name sc)]
        [(Fun bound-id bound-body) expr]
        [(Call fun-expr arg-expr)
         (define fval (eval fun-expr sc))
         (cases fval
           [(Fun bound-id bound-body)
            (eval bound-body (extend bound-id (eval arg-expr sc) sc))]
           [else (error 'eval "`call' expects a function, got: ~s"
                        fval)])]))

    (: run : String -> Number)
    ;; evaluate a FLANG program contained in a string
    (define (run str)
      (let ([result (eval (parse str) empty-subst)])
        (cases result
          [(Num n) n]
          [else (error 'run "evaluation returned a non-number: ~s"
                       result)])))

    ;; tests
    (test (run "{call {fun {x} {+ x 1}} 4}")
          => 5)
    (test (run "{with {add3 {fun {x} {+ x 3}}}
                  {call add3 1}}")
          => 4)
    (test (run "{with {add3 {fun {x} {+ x 3}}}
                  {with {add1 {fun {x} {+ x 1}}}
                    {with {x 3}
                      {call add1 {call add3 x}}}}}")
          => 7)
    (test (run "{with {identity {fun {x} x}}
                  {with {foo {fun {x} {+ x 1}}}
                    {call {call identity foo} 123}}}")
          => 124)
    (test (run "{call {with {x 3}
                        {fun {y} {+ x y}}}
                      4}")
          => 7)
    (test (run "{with {f {with {x 3} {fun {y} {+ x y}}}}
                  {with {x 100}
                    {call f 4}}}")
          => 7)
    (test (run "{with {x 3}
                  {with {f {fun {y} {+ x y}}}
                    {with {x 5}
                      {call f 4}}}}")
          => "???")
    (test (run "{call {call {fun {x} {call x 1}}
                            {fun {x} {fun {y} {+ x y}}}}
                      123}")
          => 124)


------------------------------------------------------------------------
# Dynamic and Lexical Scopes

This seems like it should work, and it even worked on a few examples,
except for one which was hard to follow. Seems like we have a bug...

Now we get to a tricky issue that managed to be a problem for *lots* of
language implementors, including the first version of Lisp. Lets try to
run the following expression --- try to figure out what it will evaluate
to:

    (run "{with {x 3}
            {with {f {fun {y} {+ x y}}}
              {with {x 5}
                {call f 4}}}}")

We expect it to return `7` (at least I do!), but we get `9` instead...
The question is --- *should* it return `9`?

What we have arrived to is called *dynamic scope*. Scope is determined
by the dynamic run-time environment (which is represented by our
substitution cache). This is *almost always* undesirable, as I hope to
convince you.

Before we start, we define two scope options for a programming language:

* Static Scope (also called Lexical Scope): In a language with static
  scope, each identifier gets its value from the scope of its
  definition, not its use.

* Dynamic Scope: In a language with dynamic scope, each identifier gets
  its value from the scope of its use, not its definition.

Racket uses lexical scope, our new evaluator uses dynamic, the old
substitution-based evaluator was static etc.

As a side-remark, Lisp began its life as a dynamically-scoped language.
The artifacts of this were (sort-of) dismissed as an implementation bug.
When Scheme was introduced, it was the first Lisp dialect that used
strictly lexical scoping, and Racket is obviously doing the same. (Some
Lisp implementations used dynamic scope for interpreted code and lexical
scope for compiled code!) In fact, Emacs Lisp is the only *live*
dialects of Lisp that is still dynamically scoped by default. To see
this, compare a version of the above code in Racket:

    (let ((x 3))
      (let ((f (lambda (y) (+ x y))))
        (let ((x 5))
          (f 4))))

and the Emacs Lisp version (which looks almost the same):

    (let ((x 3))
      (let ((f (lambda (y) (+ x y))))
        (let ((x 5))
          (funcall f 4))))

which also happens when we use another function on the way:

    (defun blah (func val)
      (funcall func val))

    (let ((x 3))
      (let ((f (lambda (y) (+ x y))))
        (let ((x 5))
          (blah f 4))))

and note that renaming identifiers can lead to different code --- change
that `val` to `x`:

    (defun blah (func x)
      (funcall func x))

    (let ((x 3))
      (let ((f (lambda (y) (+ x y))))
        (let ((x 5))
          (blah f 4))))

and you get `8` because the argument name changed the `x` that the
internal function sees!

Consider also this Emacs Lisp function:

    (defun return-x ()
      x)

which has no meaning by itself (`x` is unbound),

    (return-x)

but can be given a dynamic meaning using a `let`:

    (let ((x 5)) (return-x))

or a function application:

    (defun foo (x)
      (return-x))

    (foo 5)

There is also a dynamically-scoped language in the course languages:

    #lang pl dynamic

    (define x 123)

    (define (getx) x)

    (define (bar1 x) (getx))
    (define (bar2 y) (getx))

    (test (getx) => 123)
    (test (let ([x 456]) (getx)) => 456)
    (test (getx) => 123)
    (test (bar1 999) => 999)
    (test (bar2 999) => 123)

    (define (foo x) (define (helper) (+ x 1)) helper)
    (test ((foo 0)) => 124)

    ;; and *much* worse:
    (define (add x y) (+ x y))
    (test (let ([+ *]) (add 6 7)) => 42)

Note how bad the last example gets: you basically cannot call any
function and know in advance what it will do.

There are some cases where dynamic scope can be useful in that it allows
you to "remotely" customize any piece of code. A good example of where
this is taken to an extreme is Emacs: originally, it was based on an
ancient Lisp dialect that was still dynamically scoped, but it retained
this feature even when practically all Lisp dialects moved on to having
lexical scope by default. The reason for this is that the danger of
dynamic scope is also a way to make a very open system where almost
anything can be customized by changing it "remotely". Here's a concrete
example for a similar kind of dynamic scope usage that makes a very
hackable and open system:

    #lang pl dynamic

    (define tax% 6.25)
    (define (with-tax n)
      (+ n (* n (/ tax% 100))))

    (with-tax 10) ; how much do we pay?
    (let ([tax% 17.0]) (with-tax 10)) ; how much would we pay in Israel?

    ;; make that into a function
    (define il-tax% 17.0)
    (define (ma-over-il-saving n)
      (- (let ([tax% il-tax%]) (with-tax n))
         (with-tax n)))

    (ma-over-il-saving 10)
    ;; can even control that: how much would we save if
    ;; the tax in israel went down one percent?
    (let ([il-tax% (- il-tax% 1)]) (ma-over-il-saving 10))

    ;; or change both: how much savings in NH instead of MA?
    (let ((tax% 0.0) (il-tax% tax%))
      (ma-over-il-saving 1000))

Obviously, this power to customize everything is also the main source of
problems with getting no guarantees for code. A common way to get the
best of both worlds is to have *controllable* dynamic scope. For
example, Common Lisp also has lexical scope everywhere by default, but
some variables can be declared as *special*, which means that they are
dynamically scoped. The main problem with that is that you can't tell
when a variable is special by just looking at the code that uses it, so
a more popular approach is the one that is used in Racket: all bindings
are always lexically scoped, but there are *parameters* which are a kind
of dynamically scoped value containers --- but they are bound to plain
(lexically scoped) identifiers. Here's the same code as above,
translated to Racket with parameters:

    #lang racket

    (define tax% (make-parameter 6.5))  ; create the dynamic container
    (define (with-tax n)
      (+ n (* n (/ (tax%) 100))))       ; note how its value is accessed

    (with-tax 10) ; how much do we pay?
    (parameterize ([tax% 17.0]) (with-tax 10)) ; not a `let'

    ;; make that into a function
    (define il-tax% (make-parameter 17.0))
    (define (ma-over-il-saving n)
      (- (parameterize ([tax% (il-tax%)]) (with-tax n))
         (with-tax n)))

    (ma-over-il-saving 10)
    (parameterize ([il-tax% (- (il-tax%) 1)]) (ma-over-il-saving 10))

The main point here is that the points where a dynamically scoped value
is used are under the programmer's control --- you cannot "customize"
what `-` is doing, for example. This gives us back the guarantees that
we like to have (= that code works), but of course these points are
pre-determined, unlike an environment where everything can be customized
including things that are unexpectedly useful.

> As a side-note, after many decades of debating this, Emacs has finally
> added lexical scope in its core language, but this is still determined
> by a flag --- a global `lexical-binding` variable.