Lecture #6, Tuesday, January 23rd
=================================
 Evaluation of `with` (contd.)
 Formal Specs
 Lazy vs Eager Evaluation
 de Bruijn Indexes
 Functions & Function Values
 Implementing First Class Functions
 Sidenote: how important is it to have *anonymous* functions?

# Evaluation of `with` (contd.)
Reminder:
* We started doing substitution, with a `let`like form: `with`.
* Reasons for using bindings:
 Avoid writing expressions twice.
* More expressive language (can express identity).
* Duplicating is bad! ("DRY": *Don't Repeat Yourself*.)
* Avoids *static* redundancy.
 Avoid redundant computations.
* More than *just* an optimization when it avoids exponential
resources.
* Avoids *dynamic* redundancy.
* BNF:
::=
 { + }
 {  }
 { * }
 { / }
 { with { } }

Note that we had to introduce two new rules: one for introducing an
identifier, and one for using it.
* Type definition:
(definetype WAE
[Num Number]
[Add WAE WAE]
[Sub WAE WAE]
[Mul WAE WAE]
[Div WAE WAE]
[Id Symbol]
[With Symbol WAE WAE])
* Parser:
(: parsesexpr : Sexpr > WAE)
;; parses sexpressions into WAEs
(define (parsesexpr 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 (parsesexpr named) (parsesexpr body))]
[else (error 'parsesexpr "bad `with' syntax in ~s"
sexpr)])]
[(list '+ lhs rhs) (Add (parsesexpr lhs) (parsesexpr rhs))]
[(list ' lhs rhs) (Sub (parsesexpr lhs) (parsesexpr rhs))]
[(list '* lhs rhs) (Mul (parsesexpr lhs) (parsesexpr rhs))]
[(list '/ lhs rhs) (Mul (parsesexpr lhs) (parsesexpr rhs))]
[else (error 'parsesexpr "bad syntax in ~s" sexpr)]))
* We need to define substitution. Terms:
1. Binding Instance.
2. Scope.
3. Bound Instance.
4. Free Instance.
* After lots of attempts:
> e[v/i]  To substitute an identifier `i` in an expression `e`
> with an expression `v`, replace all instances of `i` that are free
> in `e` with the expression `v`.
* Implemented the code, and again, needed to fix a few bugs:
(: subst : WAE Symbol WAE > WAE)
;; substitutes the second argument with the third argument in the
;; first argument, as per the rules of substitution; the resulting
;; expression contains no free instances of the second argument
(define (subst expr from to)
(cases expr
[(Num n) expr]
[(Add l r) (Add (subst l from to) (subst r from to))]
[(Sub l r) (Sub (subst l from to) (subst r from to))]
[(Mul l r) (Mul (subst l from to) (subst r from to))]
[(Div l r) (Div (subst l from to) (subst r from to))]
[(Id name) (if (eq? name from) to expr)]
[(With boundid namedexpr boundbody)
(With boundid
(subst namedexpr from to)
(if (eq? boundid from)
boundbody
(subst boundbody from to)))]))
(Note that the bugs that we fixed clarify the exact way that our
scopes work: in `{with {x 2} {with {x {+ x 2}} x}}`, the scope of the
first `x` is the `{+ x 2}` expression.)
* We then extended the AE evaluation rules:
eval(...) = ... same as the AE rules ...
eval({with {x E1} E2}) = eval(E2[eval(E1)/x])
eval(id) = error!
and noted the possible type problem.
* The above translated into a Racket definition for an `eval` function
(with a hack to avoid the type issue):
(: eval : WAE > Number)
;; evaluates WAE expressions by reducing them to numbers
(define (eval expr)
(cases expr
[(Num n) n]
[(Add l r) (+ (eval l) (eval r))]
[(Sub l r) ( (eval l) (eval r))]
[(Mul l r) (* (eval l) (eval r))]
[(Div l r) (/ (eval l) (eval r))]
[(With boundid namedexpr boundbody)
(eval (subst boundbody
boundid
(Num (eval namedexpr))))]
[(Id name) (error 'eval "free identifier: ~s" name)]))

# Formal Specs
Note the formal definitions that were included in the WAE code. They are
ways of describing pieces of our language that are more formal than
plain English, but still not as formal (and as verbose) as the actual
code.
A formal definition of `subst`:
(`N` is a ``, `E1`, `E2` are ``s, `x` is some ``, `y` is a
*different* ``)
N[v/x] = N
{+ E1 E2}[v/x] = {+ E1[v/x] E2[v/x]}
{ E1 E2}[v/x] = { E1[v/x] E2[v/x]}
{* E1 E2}[v/x] = {* E1[v/x] E2[v/x]}
{/ E1 E2}[v/x] = {/ E1[v/x] E2[v/x]}
y[v/x] = y
x[v/x] = v
{with {y E1} E2}[v/x] = {with {y E1[v/x]} E2[v/x]}
{with {x E1} E2}[v/x] = {with {x E1[v/x]} E2}
And a formal definition of `eval`:
eval(N) = N
eval({+ E1 E2}) = eval(E1) + eval(E2)
eval({ E1 E2}) = eval(E1)  eval(E2)
eval({* E1 E2}) = eval(E1) * eval(E2)
eval({/ E1 E2}) = eval(E1) / eval(E2)
eval(id) = error!
eval({with {x E1} E2}) = eval(E2[eval(E1)/x])

# Lazy vs Eager Evaluation
As we have previously seen, there are two basic approaches for
evaluation: either eager or lazy. In lazy evaluation, bindings are used
for sort of textual references  it is only for avoiding writing an
expression twice, but the associated computation is done twice anyway.
In eager evaluation, we eliminate not only the textual redundancy, but
also the computation.
Which evaluation method did our evaluator use? The relevant piece of
formalism is the treatment of `with`:
eval({with {x E1} E2}) = eval(E2[eval(E1)/x])
And the matching piece of code is:
[(With boundid namedexpr boundbody)
(eval (subst boundbody
boundid
(Num (eval namedexpr))))]
How do we make this lazy?
In the formal equation:
eval({with {x E1} E2}) = eval(E2[E1/x])
and in the code:
(: eval : WAE > Number)
;; evaluates WAE expressions by reducing them to numbers
(define (eval expr)
(cases expr
[(Num n) n]
[(Add l r) (+ (eval l) (eval r))]
[(Sub l r) ( (eval l) (eval r))]
[(Mul l r) (* (eval l) (eval r))]
[(With boundid namedexpr boundbody)
(eval (subst boundbody
boundid
namedexpr))] ;*** no eval and no Num wrapping
[(Id name) (error 'eval "free identifier: ~s" name)]))
We can verify the way this works by tracing `eval` (compare the trace
you get for the two versions):
> (trace eval) ; (put this in the definitions window)
> (run "{with {x {+ 1 2}} {* x x}}")
Ignoring the traces for now, the modified WAE interpreter works as
before, specifically, all tests pass. So the question is whether the
language we get is actually different than the one we had before. One
difference is in execution speed, but we can't really notice a
difference, and we care more about meaning. Is there any program that
will run differently in the two languages?
The main feature of the lazy evaluator is that it is not evaluating the
named expression until it is actually needed. As we have seen, this
leads to duplicating computations if the bound identifier is used more
than once  meaning that it does not eliminate the dynamic redundancy.
But what if the bound identifier is not used at all? In that case the
named expression simply evaporates. This is a good hint at an expression
that behaves differently in the two languages  if we add division to
both languages, we get a different result when we try running:
{with {x {/ 8 0}} 7}
The eager evaluator stops with an error when it tries evaluating the
division  and the lazy evaluator simply ignores it.
Even without division, we get a similar behavior for
{with {x y} 7}
but it is questionable whether the fact that this evaluates to 7 is
correct behavior  we really want to forbid program that use free
variable.
Furthermore, there is an issue with name capturing  we don't want to
substitute an expression into a context that captures some of its free
variables. But our substitution allows just that, which is usually not a
problem because by the time we do the substitution, the named expression
should not have free variables that need to be replaced. However,
consider evaluating this program:
{with {y x}
{with {x 2}
{+ x y}}}
under the two evaluation regimens: the eager version stops with an
error, and the lazy version succeed. This points at a bug in our
substitution, or rather not dealing with an issue that we do not
encounter.
So the summary is: as long as the initial program is correct, both
evaluation regimens produce the same results. If a program contains free
variables, they might get captured in a naive lazy evaluator
implementation (but this is a bug that should be fixed). Also, there are
some cases where eager evaluation runs into a runtime problem which
does not happen in a lazy evaluator because the expression is not used.
It is possible to prove that when you evaluate an expression, if there
is an error that can be avoided, lazy evaluation will always avoid it,
whereas an eager evaluator will always run into it. On the other hand,
lazy evaluators are usually slower than eager evaluator, so it's a speed
vs. robustness tradeoff.
Note that with lazy evaluation we say that an identifier is bound to an
expression rather than a value. (Again, this is why the eager version
needed to wrap `eval`'s result in a `Num` and this one doesn't.)
(It is possible to change things and get a more well behaved
substitution, we basically will need to find if a capture might happen,
and rename things to avoid it. For example,
{with {y E1} E2}[v/x]
if `x' and `y' are equal
= {with {y E1[v/x]} E2} = {with {x E1[v/x]} E2}
if `y' has a free occurrence in `v'
= {with {y1 E1[v/x]} E2[y1/y][v/x]} ; `y1' is "fresh"
otherwise
= {with {y E1[v/x]} E2[v/x]}
With this, we might have gone through this path in evaluating the above:
{with {y x} {with {x 2} {+ x y}}}
{with {x₁ 2} {+ x₁ x}} ; note that x₁ is a fresh name, not x
{+ 2 x}
error: free `x`
But you can see that this is much more complicated (more code: requires
a `freein` predicate, being able to invent new *fresh* names, etc). And
it's not even the end of that story...)

# de Bruijn Indexes
This whole story revolves around names, specifically, name capture is a
problem that should always be avoided (it is one major source of PL
headaches).
But are names the only way we can use bindings?
There is a least one alternative way: note that the only thing we used
names for are for references. We don't really care what the name is,
which is pretty obvious when we consider the two WAE expressions:
{with {x 5} {+ x x}}
{with {y 5} {+ y y}}
or the two Racket function definitions:
(define (foo x) (list x x))
(define (foo y) (list y y))
Both of these show a pair of expressions that we should consider as
equal in some sense (this is called "alphaequality"). The only thing we
care about is what variable points where: the binding structure is the
only thing that matters. In other words, as long as DrRacket produces
the same arrows when we use Check Syntax, we consider the program to be
the same, regardless of name choices (for argument names and local
names, not for global names like `foo` in the above).
The alternative idea uses this principle: if all we care about is where
the arrows go, then simply get rid of the names... Instead of
referencing a binding through its name, just specify which of the
surrounding scopes we want to refer to. For example, instead of:
{with {x 5} {with {y 6} {+ x y}}}
we can use a new "reference" syntax  `[N]`  and use this instead
of the above:
{with 5 {with 6 {+ [1] [0]}}}
So the rules for `[N]` are  `[0]` is the value bound in the current
scope, `[1]` is the value from the next one up etc.
Of course, to do this translation, we have to know the precise scope
rules. Two more complicated examples:
{with {x 5} {+ x {with {y 6} {+ x y}}}}
is translated to:
{with 5 {+ [0] {with 6 {+ [1] [0]}}}}
(note how `x` appears as a different reference based on where it
appeared in the original code.) Even more subtle:
{with {x 5} {with {y {+ x 1}} {+ x y}}}
is translated to:
{with 5 {with {+ [0] 1} {+ [1] [0]}}}
because the inner `with` does not have its own named expression in its
scope, so the named expression is immediately in the scope of the outer
`with`.
This is called "de Bruijn Indexes": instead of referencing identifiers
by their name, we use an index into the surrounding binding context. The
major disadvantage, as can be seen in the above examples, is that it is
not convenient for humans to work with. Specifically, the same
identifier is referenced using different numbers, which makes it hard to
understand what some code is doing. After all, *abstractions* are the
main thing we deal with when we write programs, and having labels make
the bindings structure much easier to understand than scope counts.
However, practically all compilers use this for compiled code (think
about stack pointers). For example, GCC compiles this code:
{
int x = 5;
{
int y = x + 1;
return x + y;
}
}
to:
subl $8, %esp
movl $5, 4(%ebp) ; int x = 5
movl 4(%ebp), %eax
incl %eax
movl %eax, 8(%ebp) ; int y = %eax
movl 8(%ebp), %eax
addl 4(%ebp), %eax

# Functions & Function Values
> [PLAI §4]
Now that we have a form for local bindings, which forced us to deal with
proper substitutions and everything that is related, we can get to
functions. The concept of a function is itself very close to
substitution, and to our `with` form. For example, when we write:
{with {x 5}
{* x x}}
then the `{* x x}` body is itself parametrized over some value for `x`.
If we take this expression and take out the `5`, we're left with
something that has all of the necessary ingredients of a function  a
bunch of code that is parameterized over some input identifier:
{with {x}
{* x x}}
We only need to replace `with` and use a proper name that indicates that
it's a function:
{fun {x}
{* x x}}
Now we have a new form in our language, one that should have a function
as its meaning. As we have seen in the case of `with` expressions, we
also need a new form to *use* these functions. We will use `call` for
this, so that
{call {fun {x} {* x x}}
5}
will be the same as the original `with` expression that we started with
 the `fun` expression is like the `with` expression with no value,
and applying it on `5` is providing that value back:
{with {x 5}
{* x x}}
Of course, this does not help much  all we get is a way to use local
bindings that is more verbose from what we started with. What we're
really missing is a way to *name* these functions. If we get the right
evaluation rules, we can evaluate a `fun` expression to some value 
which will allow us to bind it to a variable using `with`. Something
like this:
{with {sqr {fun {x} {* x x}}}
{+ {call sqr 5}
{call sqr 6}}}
In this expression, we say that `x` is the formal parameter (or
argument), and the `5` and `6` are actual parameters (sometimes
abbreviated as formals and actuals). Note that naming functions often
helps, but many times there are small functions that are fine to specify
without a name  for example, consider a twostage addition function,
where there is no apparent good name for the returned function:
{with {add {fun {x}
{fun {y}
{+ x y}}}}
{call {call add 8} 9}}

# Implementing First Class Functions
> [PLAI §6] (uses some stuff from [PLAI §5], which we do later)
This is a simple plan, but it is directly related to how functions are
going to be used in our language. We know that `{call {fun {x} E1} E2}`
is equivalent to a `with` expression, but the new thing here is that we
do allow writing just the `{fun ...}` expression by itself, and
therefore we need to have some meaning for it. The meaning, or the value
of this expression, should roughly be "an expression that needs a value
to be plugged in for `x`". In other words, our language will have these
new kinds of values that contain an expression to be evaluated later on.
There are three basic approaches that classify programming languages in
relation to how the deal with functions:
1. First order: functions are not real values. They cannot be used or
returned as values by other functions. This means that they cannot be
stored in data structures. This is what most "conventional" languages
used to have in the past. (You will be implementing such a language
in homework 4.)
An example of such a language is the Beginner Student language that
is used in HtDP, where the language is intentionally firstorder to
help students write correct code (at the early stages where using a
function as a value is usually an error). It's hard to find practical
modern languages that fall in this category.
2. Higher order: functions can receive and return other functions as
values. This is what you get with C and modern Fortran.
3. First class: functions are values with all the rights of other
values. In particular, they can be supplied to other functions,
returned from functions, stored in data structures, and new functions
can be created at runtime. (And most modern languages have first
class functions.)
The last category is the most interesting one. Back in the old days,
complex expressions were not firstclass in that they could not be
freely composed. This is still the case in machinecode: as we've seen
earlier, to compute an expression such as
(b + sqrt(b^2  4*a*c)) / 2a
you have to do something like this:
x = b * b
y = 4 * a
y = y * c
x = x  y
x = sqrt(x)
y = b
x = y + x
y = 2 * a
s = x / y
In other words, every intermediate value needs to have its own name. But
with proper ("highlevel") programming languages (at least most of
them...) you can just write the original expression, with no names for
these values.
With firstclass functions something similar happens  it is possible
to have complex expressions that consume and return functions, and they
do not need to be named.
What we get with our `fun` expression (if we can make it work) is
exactly this: it generates a function, and you can choose to either bind
it to a name, or not. The important thing is that the value exists
independently of a name.
This has a major effect on the "personality" of a programming language
as we will see. In fact, just adding this feature will make our language
much more advanced than languages with just higherorder or firstorder
functions.

Quick Example: the following is working JavaScript code, that uses first
class functions.
function foo(x) {
function bar(y) { return x + y; }
return bar;
}
function main() {
var f = foo(1);
var g = foo(10);
return [f(2), g(2)];
}
Note that the above definition of `foo` does *not* use an anonymous
"lambda expression"  in Racket terms, it's translated to
(define (foo x)
(define (bar y) (+ x y))
bar)
The returned function is not anonymous, but it's not really named
either: the `bar` name is bound only inside the body of `foo`, and
outside of it that name no longer exists since it's not its scope. It
gets used in the printed form if the function value is displayed, but
this is merely a debugging aid. The anonymous `lambda` version that is
common in Racket can be used in JavaScript too:
function foo(x) {
return function(y) { return x + y; }
}
> Sidenote: GCC includes extensions that allow internal function
> definitions, but it still does not have first class functions 
> trying to do the above is broken:
>
> #include
> typedef int(*int2int)(int);
> int2int foo(int x) {
> int bar(int y) { return x + y; }
> return bar;
> }
> int main() {
> int2int f = foo(1);
> int2int g = foo(10);
> printf(">> %d, %d\n", f(2), g(2));
> }

## Sidenote: how important is it to have *anonymous* functions?
You'll see many places where people refer to the feature of firstclass
functions as the ability to create *anonymous* functions, but this is a
confusion and it's not accurate. Whether a function has a name or not is
not the important question  instead, the important question is
whether functions can exist with no *bindings* that refers to them.
As a quick example in Racket:
(define (foo x)
(define (bar y) (+ x y))
bar)
in Javascript:
function foo(x) {
function bar(y) {
return x + y;
}
return bar;
}
and in Python:
def foo(x):
def bar(y):
return x + y
return bar
In all three of these, we have a `foo` function that returns a function
*named* `bar`  but the `bar` name, is only available in the scope of
`foo`. The fact that the name is displayed as part of the textual
rendering of the function value is merely a debugging feature.