Introduction

This library allows macro writers to provide better feedback to macro users when errors are signaled during macroexpansion.

Note: While this library can be loaded into and used in any Common Lisp implementation, the improved behavior described below is only available in CLASP and SBCL (only in versions 1.3.13 and newer).

For example, consider the following macro

(defmacro even-number-case (value &body clauses)
  "Like `cl:case' but each key has to be an even number."
  (alexandria:once-only (value)
    `(cond ,@(mapcar (lambda (clause)
                       (destructuring-bind (number &rest body) clause
                         (unless (evenp number)
                           (error "Only even numbers are allowed."))
                         `((= ,value ,number)
                           ,@body)))
                     clauses))))

This is fine if the expansion does not signal an error. If it does, however, it is not immediately clear which of the clauses (if any) caused the error:

(defun foo (x)
  (even-number-case x
    (2 :two)
    (4 :four)
    (5 :fix)
    (8 :eight)
    (10 :ten)))

The problem is not very hard to spot in the above code, but think of macros for declaring complex things like cl:defpackage or cl:defclass or macros for domain specific languages, and the problem becomes more severe.

The mechanism provided by this library is the with-current-source-form macro. The macro is intended to surround parts of macro expanders that process certain sub-forms of the form passed to the expander:

(defmacro even-number-case (value &body clauses)
  "Like `cl:case' but each key has to be an even number."
  (alexandria:once-only (value)
    `(cond ,@(mapcar (lambda (clause)
                       (trivial-with-current-source-form:with-current-source-form (clause)
                         (destructuring-bind (number &rest body) clause
                           (unless (evenp number)
                             (error "Only even numbers are allowed."))
                           `((= ,value ,number)
                             ,@body))))
                     clauses))))

The effect of the above change is that the implementation can now report a more useful location when reporting the error during macro expansion. Other tools like SLIME benefit from this functionality as well:

Tutorial

Reference

with-current-source-form (FORM &REST FORMS) &BODY BODY

In a macroexpander, indicate that FORM, FORMS are being processed by BODY.

FORMS are usually sub-forms of the whole form passed to the expander.

If more than one form is supplied, FORMS should be ordered by
specificity, with the most specific form first. This allows the
compiler to try and obtain a source path using subsequent elements of
FORMS if it fails for the first one.

Indicating the processing of sub-forms lets the compiler report
precise source locations in case conditions are signaled during the
execution of BODY.