# Assignment 6 (week 7)
## Evaluation order in Combinatory Logic
1. Give a term that the lazy evaluators (either [[the Haskell
evaluator|code/ski_evaluator.hs]], or the lazy version of [[the OCaml
evaluator|code/ski_evaluator.ml]]) do not evaluate all the way to a
normal form, i.e., that contains a redex somewhere inside of it after
it has been reduced.
2. One of the [[criteria we established for classifying reduction
strategies|topics/week3_evaluation_order]] strategies is whether they
reduce subexpressions hidden under lambdas. That is, for a term like
`(\x y. x z) (\x. x)`, do we reduce to `\y.(\x.x) z` and stop, or do
we reduce further to `\y.z`? Explain what the corresponding question
would be for CL. Using either the OCaml CL evaluator or the Haskell
evaluator developed in the wiki notes, prove that the evaluator does
reduce expressions inside of at least some "functional" CL
expressions. Then provide a modified evaluator that does not perform
reductions in those positions. (Just give the modified version of your
recursive reduction function.)
## Evaluation in the untyped lambda calculus: substitution
Once you grok reduction and evaluation order in Combinatory Logic,
we're going to begin to construct an evaluator for a simple language
that includes lambda abstraction. We're going to work through the
issues twice: once with a function that does substitution in the
obvious way. You'll see it's somewhat complicated. The complications
come from the need to worry about variable capture. (Seeing these
complications should give you an inkling of why we presented the
evaluation order discussion using Combinatory Logic, since we don't
need to worry about variables in CL.)
We're not going to ask you to write the entire program yourself.
Instead, we're going to give you [[the complete program, minus a few
little bits of glue|code/reduction_with_substitution.ml]]. What you need to do is
understand how it all fits together. When you do, you'll understand
how to add the last little bits to make functioning program.
1. In the previous homework, you built a function that took an
identifier and a lambda term and returned a boolean representing
whether that identifier occured free inside of the term. Your first
task is to complete the `free_in` function, which has been crippled in
the code base (look for lines that say `COMPLETE THIS LINE`). Once
you have your function working, you should be able to run queries such
as this:
# free_in "x" (App (Abstract ("x", Var "x"), Var "x"));;
- : bool = true
2. Once you get the `free_in` function working, you'll need to
complete the `substitute` function. You'll see a new wrinkle on
OCaml's pattern-matching construction: `| PATTERN when x = 2 ->
RESULT`. This means that a match with PATTERN is only triggered if
the boolean condition in the `when` clause evaluates to true.
Sample target:
# substitute (App (Abstract ("x", ((App (Abstract ("x", Var "x"), Var "y")))), Constant (Num 3))) "y" (Constant (Num 4));;
- : lambdaTerm = App (Abstract ("x", App (Abstract ("x", Var "x"), Constant (Num 4))), Constant (Num 3))
3. Once you have completed the previous two problems, you'll have a
complete evaluation program. Here's a simple sanity check for when you
get it working:
# reduce (App (Abstract ("x", Var "x"), Constant (Num 3)));;
- : lambdaTerm = Constant (Num 3)
4. What kind of evaluation strategy does this evaluator use? In
particular, what are the answers to the three questions about
evaluation strategy as given in the discussion of [[evaluation
strategies|topics/week3_evaluation_order]] as Q1, Q2, and Q3?
## Evaluation in the untyped calculus: environments
Ok, the previous strategy sucked: tracking free and bound variables,
computing fresh variables, it's all super complicated.
Here's a better strategy. Instead of keeping all of the information
about which variables have been bound or are still free implicitly
inside of the terms, we'll keep score. This will require us to carry
around a scorecard, which we will call an "environment". This is a
familiar strategy, since it amounts to evaluating expressions relative
to an assignment function. The difference between the assignment
function approach above, and this approach, is one huge step towards
monads.
5. First, you need to get [[the evaluation
code|code/reduction_with_environments.ml]] working. Look in the
code for places where you see "not yet implemented", and get enough of
those places working that you can use the code to evaluate terms.
6. A snag: what happens when we want to replace a variable with a term
that itself contains a free variable?
term environment
------------- -------------
(\w.(\y.y)w)2 []
(\y.y)w [w->2]
y [w->2, y->w]

In the first step, we bind `w` to the argument `2`. In the second
step, we bind `y` to the argument `w`. In the third step, we would
like to replace `y` with whatever its current value is according to
our scorecard. On the simple-minded view, we would replace it with
`w`. But that's not the right result, because `w` itself has been
mapped onto 2.