Questions

  • Homework questions?
  • Anyone else gotten started on the one from yesterday?
  • Look for test code

Parameter Passing Conventions

Today we implement 4, 4 interpreters, which exhibit different parameter-passing conventions.

Preludes: Boxes

> (box 'cat)
#&cat
> (define b (box 'cat))
> b
#&cat
> (unbox b)
cat
> b
#&cat
> (set-box! b 'dog)
> b
#&dog
>

All problems in computer science can be solved by another level of indirection. – David Wheeler

Call-by-value interpreter, revisited

We will at first revisit this interpreter, and a short-circuit line. This is for a derivation, not initially motivated in and of itself. But you could imagine wanting this, if you know this situation happens often.

Call-by-reference interpreter

Notice!

> (define b (box 'cat))
> (define c (box (unbox b)))
> (eqv? b c)
#f

Call-by-name interpreter

Call-by-need interpreter

  Strict evaluation Non-strict evaluation
Passes new box CBV (the standard)  
Passes same box CBReference CBName

And our call-by-need interpreter is a variation on the CBName one, that uses the side-effect for efficiency.

See both your own lecture notes, and also our linked PDF document deriving these four interpreters to refresh and recapitulate.

Updated: