[r6rs-discuss] [Formal] declarations break lexical scoping

Mon Oct 23 08:49:08 EDT 2006

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Name        : Andre van Tonder
Email       : andre at het.brown.edu
Type        : defect
Priority    : major
Component   : Bodies and sequences (9.4) and Expansion process (8)
Version     : 5.91
Pages       : 28, 29 and 30
Dependencies: None

Summary:
--------

Expansion process as currently described violates lexical scoping
for bodies with "declare" forms.

Description:
------------

Consider

Example 1:

   (let ()
     (declare safe)
     (define declare 1))

Example 2:

   (let ()
     (declare safe)
     (define safe 1))

According to the algorith in the first paragraph of p.28, it appears that the 
first line in both examples should be accepted as a declaration.  This is in 
conflict with lexical scoping according to the second paragraph of p.30, which 
says that "the region of the binding is the entire <body>"

Suggestion:
-----------

I see two options:

Option 1:
---------
Example 1 (but not example 2) can be fixed by changing the line on p 28:

  "To detect this error, the expander records the
  identifying keyword for each macro use, define-syntax
  form, define form, begin form, let-syntax form, and
  letrec-syntax form it encounters while processing the
  definitions ..."

  -->

  "To detect this error, the expander records the
  identifying keyword for each macro use, [[declare form]], define-syntax
  form, define form, begin form, let-syntax form, and
  letrec-syntax form it encounters while processing the
  [[declarations and]] definitions ..."

  Example 2 could be "fixed" by requiring declarations' <declare spec>s
  to be expanded after all definitions have been discovered, along with
  right hand sides and expressions.  This has a problem:

  The problem with this second part of the fix is that by the time
  all definitions have been discovered, some expressions
  (all of them in case of a script, and at least the first one in
  case of other bodies) have already been partially expanded.
  Since declarations presumably might be intended to affect the
  result of expanding expressions (e.g., by expanding to safe or
  unsafe primitives), this might break the declarations.

Option 2:
---------
Instead of the scope of a definition being the whole
<body>, the scope could be defined to be just the

   <definition>...<sequence>...

part of the body (for this to be consistent also with the expansion
process for scripts, I think one would also have to require declarations to 
precede definitions in scripts).