Forum: Editorial

References: CLHS 3.4.4, CLHS 3.4.5, DESTRUCTURING-BIND, CLHS term tree-structure

Category: CLARIFICATION/CHANGE

Edit history: 2004-07-20, Version 1 by Bruno Haible

Status: For CLiki consideration

Problem Description:

There is confusion about the meaning of a NIL element in a macro lambda list and destructuring lambda list. One should confirm or disconfirm that NIL in a destructuring lambda list (or a macro lambda list) acts as a "don't care" symbol, matching any object without causing a binding.

Proposal (DESTRUCTURING-NIL:DISALLOW):

Make it explicit that NIL as element of a macro lambda list or destructuring lambda list that has no lambda list keywords denotes the variable NIL, which is not useful since NIL being a constant, it cannot be bound or assigned.

Proposal (DESTRUCTURING-NIL:ATOMIC):

Make it explicit that NIL as element of a macro lambda list or destructuring lambda list that has no lambda list keywords is allowed and matches only the NIL object, but produces no variable bindings.

Proposal (DESTRUCTURING-NIL:WILDCARD):

Make it explicit that NIL as element of a macro lambda list or destructuring lambda list that has no lambda list keywords matches any object, but produces no variable bindings.

Test case:

(macrolet ((%m (nil) :good)) (%m nil))
(macrolet ((%m (nil) :good)) (%m t))

Rationale:

CLHS 3.4.4 points to ATOMIC, since "tree structure" distinguish between cons and atom, and NIL is an atom.

Current practice:

CLISP has been implementing DISALLOW for years and has now switched to ATOMIC.

Cost to Implementors:

Change the handling of macro- and destructuring lambda lists.

Cost to Users:

None, since relying on any of these behaviours has already been obscure.

Cost of Non-Adoption:

Continued confusion.

Benefits:

Clarity.

Aesthetics:

Discussion:

• Bruno Haible votes for ATOMIC, since it preserves the symmetry between the destructuring lambda lists (X . NIL) and (NIL . X), while the other proposals don't.
• Kalle Olavi Niemitalo says about WILDCARD: Why would it do that, other than in LOOP destructuring?