README.org

#+OPTIONS: toc:nil num:nil

• Introduction

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

  #+BEGIN_QUOTE Note: While this library can be loaded into and used in any Common Lisp implementation, the improved behavior described below is only available in [[https://github.com/clasp-developers/clasp][CLASP]] and [[http://www.sbcl.org][SBCL]] (only in versions 1.3.13 and newer). #+END_QUOTE

  For example, consider the following macro

  #+BEGIN_SRC lisp (defmacro even-number-case (value &body clauses) "Like cl:case' but each key has to be an even number." (let ((value-var (gensym "VALUE"))) (let ((,value-var ,value)) (cond ,@(mapcar (lambda (clause) (destructuring-bind (number &rest body) clause (unless (evenp number) (error "Only even numbers are allowed.")) `((= ,value-var ,number) ,@body))) clauses))))) #+END_SRC

  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:

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

  [[file:pictures/bad-expansion-error.png]]

  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:

  #+BEGIN_SRC lisp (defmacro even-number-case (value &body clauses) "Like cl:case' but each key has to be an even number." (let ((value-var (gensym "VALUE"))) (let ((,value-var ,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-var ,number) ,@body)))) clauses))))) #+END_SRC

  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:

  [[file:pictures/better-expansion-error.png]]

• Tutorial

  Since the example given in the [[*Introduction]] should explain the basic usage of this library, here are just a few additional hints:

  • trivial-with-current-source-form:with-current-source-form optionally accepts additional source forms besides the mandatory one. The reason for this mechanism is that a Common Lisp implementation may be unable to produce a source location for the most specific source form, for example if that form is a symbol or a number. In such cases, the client may be able to help the implementation by providing additional, less specific source forms which contain the first form as sub-forms.

    For example, if a macro expansion function detects a problem with foo (bound to, say, head in the expansion function) in (foo bar baz) (bound to call in the expansion function), the expansion function could provide the source information as

    #+BEGIN_SRC (trivial-with-current-source-form:with-current-source-form (head call) …) #+END_SRC

    in case the implementation cannot handle just head.

• Reference

  #+BEGIN_SRC lisp :results none :exports none #.(progn #1=(ql:quickload '("trivial-with-current-source-form")) '#1#) (defun doc (symbol kind) (let* ((lambda-list (sb-introspect:function-lambda-list symbol)) (string (documentation symbol kind)) (lines (loop :for start = 0 :then (1+ end) :for end = (position #\Newline string :start start) :while end :collect (subseq string start end) :into lines :finally (return (nconc lines (list (subseq string start)))))) (trimmed (mapcar (lambda (line) (string-left-trim '(#\Space) line)) lines))) (format nil "(A) <{A^ }:@>2%{A^%~}" symbol (list lambda-list) trimmed))) #+END_SRC

  #+BEGIN_SRC lisp :results value :exports results (doc 'trivial-with-current-source-form:with-current-source-form 'function) #+END_SRC

  #+RESULTS: #+BEGIN_EXAMPLE 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. #+END_EXAMPLE