Transcript JDB-Lisp Version 2.0 Carine Iskander Pia Chakrabarti

JDB-Lisp Version 2.0
Carine Iskander
Pia Chakrabarti
Additional Functionality Implemented

Cadr – gets the second element by
getting the car of the cdr
Equals – checks to see whether sexp in a
list are equal
 List? – checks to see if it is a list
 Num? – checks to see if it is a num
 Atomic? – checks to see if it is atomic
 Last – returns last element of a list
 *Demonstration*

Major Scoping Problem

(let ((z 17)) (let ((z 3) (a 5) (x (lambda (x y) (- x (+ y
z))))) (let ((z 19) (a 5)) (funcall x z a))))
Static Scoping (break down by JDB himself):
1. Start with (FUNCALL X Z A)
2. Resolve X to get:
((LAMBDA (X Y) (- X (+ Y Z))) Z A)
3. Resolve Z and A in the calling scope, Scope 3 in this case. So
far we have:
((LAMBDA (X Y) (- X (+ Y Z))) 19 5)
4. Apply the parameters:
(- 19 (+ 5 Z))
5. Calculate
-3
Major Scoping Problem cont.
Looks like dynamic
scoping but it’s not!
“
FunctionEntry.call() makes an
assumption, that the SymbolTable
passed to call() represents the calling
context of the function. But the Lambda
class breaks this assumption. It
interjects a new symbol table based on
the union of the calling context and the
closure context.
”
Major Scoping Problem Fixed by JDB
Seq evaledArguments;
ArrayList<SExp> temp = new ArrayList<SExp>();
for (;arguments != null; arguments = arguments.cdr)
temp.add(arguments.car.eval(symbolTable));
evaledArguments = new Seq(temp.toArray(new
SExp[]{}));
return super.call(new
ClosureSymbolTable(symbolTable, closure),
evaledArguments);
He resolves the arguments in
the symbol tables in the proper
scope before passing them on.
But there are still problems…
This is wrong because there should be
an error when using static scoping!
How We Fixed It





We created 2 tables:
- symbol table
- closure table
If the closure table is empty,
throw an exception
Else look in the closure table
first, then look in the symbol
table for the rest
*Demonstration of something
that should work*
*Demonstration of something
that shouldn’t work*
We Also Added Dynamic Scoping

What is dynamic scoping?

We give it the most recent symbol table

*Demonstration*
Division Problem

1/9 = .1111111111111111111111111111…
Division Problem cont.


JDB’s division algorithm:
computes 1/9
if no more arguments return 1/9
else continue computation
Program errors out on first step so it never gets to
compute 9/3
How We Fixed It
We restructured the algorithm so that it
did not by default do 1/n first. However,
when we explicitly wanted it to compute
something like 1/9, it would still error out.
 To fix that problem we used a BigDecimal
division method that rounds as it goes –
which took some time to find
 *Demonstration*

What’s Left

We hoped to make static scoping and dynamic
scoping both possible in one program

We attempted writing a letd method

Due to the structure of the program, it is
impossible as of right now because every function
call is a new instance of SpecialFormsEntry class so
a flag cannot be stored and accessed properly