Helps macro writers produce better errors for macro users

trivial-with-current-source-form

#+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."
      (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))))
  #+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."
      (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))))
  #+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" "alexandria" "split-sequence"))
        '#1#)
    (defun doc (symbol kind)
      (let* ((lambda-list (sb-introspect:function-lambda-list symbol))
             (string      (documentation symbol kind))
             (lines       (split-sequence:split-sequence #\Newline string))
             (trimmed     (mapcar (alexandria:curry #'string-left-trim '(#\Space)) 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


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