SRFI 72: Hygienic macros

by André van Tonder

status: final (2005-09-21)

keywords: Syntax

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Abstract

This SRFI describes a procedural macro proposal for Scheme with the following features:

• Improved hygiene:

  We argue that conventional hygiene algorithms may lead to accidental variable capture errors in procedural macros. We propose an improved algorithm that avoids these problems.

• Reflective tower:

  We specify a reflective tower of arbitrary height, and propose a refinement of lexical scoping that takes into account the phase of use of an identifier in determining its meaning.

• Syntax-case:

  In the current proposal, the syntax-case form is expressible as a macro in terms of a simpler set of primitives and is specified as library syntax.

• Procedural interface:

  The primitive interface for manipulating compound syntax objects consists of procedures rather than special forms. In particular, the traditional abstractions car, cdr, cons , ... can be used on syntactic data.

• Fast hygiene algorithm:

  The reference implementation documents a fast imperative hygiene algorithm that is eager and linear in expression size.

• Capturing identifiers:

  A primitive make-capturing-identifier is provided for intentional variable capture and for building expansion-time fluid binding constructs.