Issue FLET-DECLARATIONS Writeup

Issue:         FLET-DECLARATIONS


References:    FLET, LABELS, MACROLET (CLtL p.113)

	       X3J13 document 86-003 item 113

	       Cleanup issue DECLARATION-SCOPE.

	       Cleanup issue DECLARE-MACROS.


Category:      ADDITION


Edit history:  Version 1, Moon, 1 Jan 1988

	       Version 2, Moon, 2 Feb 1988 (edits suggested by Masinter)


Problem description:


Declarations are not allowed before the body of FLET, LABELS, and

MACROLET, even though Common Lisp allows declarations in other seemingly

analogous places, such as LET.


Proposal (FLET-DECLARATIONS:ALLOW):


Change the syntax of FLET, LABELS, and MACROLET to allow declarations

between the list of local function/macro definitions and the body forms.


The scope of such declarations in FLET and LABELS includes the bodies

of the locally defined functions, when the declarations are pervasive.

Non-pervasive declarations have no effect on those bodies, except when

LABELS includes the body in the scope of a function non-pervasively

declared.  This paragraph follows directly from CLtL p.155 if the

locally defined function bodies are treated like initialization forms.

(This paragraph will be superseded by cleanup issue DECLARATION-SCOPE

if it passes.)


The scope of such declarations does not include the bodies of the

macro expander functions defined by MACROLET.  This is consistent with

the existing rule that the bodies of those functions are in the global

environment, not the local lexical environment.


If cleanup issue DECLARE-MACROS is not passed, in MACROLET an

invocation of one of the macros locally defined by that MACROLET is

permitted to expand into a DECLARE.


Test Cases/Examples:


(defun example (y l)

  (flet ((attach (x)

	   (setq l (append l (list x)))))

    (declare (inline attach))

    (dolist (x y)

      (unless (null (cdr x))

	(attach x)))

    l))


(example '((a apple apricot) (b banana) (c cherry) (d) (e))

	 '((1) (2) (3) (4 2) (5) (6 3 2)))

 => ((1) (2) (3) (4 2) (5) (6 3 2) (a apple apricot) (b banana) (c cherry))


The above function is erroneous in current Common Lisp.  With this

proposal, it would have an intuitively obvious meaning.


Rationale:


This will make the syntax of FLET and LET consistent.  This will make

it possible to attach declarations to function bindings; currently only

variable bindings can have attached declarations.


Current practice:


Xerox Common Lisp implements FLET-DECLARATIONS:ALLOW.

Symbolics Common Lisp does not allow declarations in this position.


Cost to Implementors:


The compilation and interpretation of three special forms will have to

be changed, however the same techniques already developed for

declarations in LET should be applicable.


Cost to Users:


No cost since this is an upward-compatible addition.


Cost of non-adoption:


Unnecessary inconsistency in the syntax of Common Lisp.


Benefits:


There is no major benefit but the language will be more consistent.


Esthetics:


Makes the language more consistent.


Discussion:


We need to resolve this for CLOS, because CLOS introduces two new

special forms similar to FLET and LABELS and we need to make their

syntax consistent with FLET and LABELS.