Evaluate any non-special args in apheleia-formatters (#55) (#56)

* Evaluate any non-special args in apheleia-formatters (#55)

Allows you to place snippets of elisp within a formatter command that
will be evaluated and substituted in place when apheleia decides to run
that command.

* Update apheleia.el

Co-authored-by: Radon Rosborough <radon.neon@gmail.com>

3 files changed, 49 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 157b3ce..07a971d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,8 @@ The format is based on [Keep a Changelog].
 * Support multiple formatters ([#31]). You can now configure a list of
   formatters for a major-mode in `apheleia-mode-alist` and they will
   be run in sequence.
+* Support evaluating items in `apheleia-formatters` to make formatter
+  commands more dynamic ([#50], [#55]).
 
 ### Formatters
 * [ClangFormat](https://clang.llvm.org/docs/ClangFormat.html) for
@@ -43,7 +45,9 @@ The format is based on [Keep a Changelog].
 [#39]: https://github.com/raxod502/apheleia/issues/39
 [#48]: https://github.com/raxod502/apheleia/pull/48
 [#49]: https://github.com/raxod502/apheleia/pull/49
+[#50]: https://github.com/raxod502/apheleia/pull/50
 [#54]: https://github.com/raxod502/apheleia/pull/54
+[#55]: https://github.com/raxod502/apheleia/issues/55
 
 ## 1.1.2 (released 2021-02-26)
 ### Enhancements
diff --git a/README.md b/README.md
index 5e84422..9292322 100644
--- a/README.md
+++ b/README.md
@@ -83,6 +83,31 @@ variables:
       (setf (alist-get 'black apheleia-formatters)
             '("black" "--option" "..." "-"))
       ```
+    * There are a list of symbols that are interpreted by apheleia
+      specially when formatting a command (example: `npx`). Any
+      non-string entries in a formatter that doesn't equal one of
+      these symbols is evaluated and replaced in place. This can be
+      used to pass certain flags to the formatter process depending on
+      the state of the current buffer. For example:
+
+      ```elisp
+      (push '(shfmt . ("beautysh"
+                       "-filename" filepath
+                       (when-let ((indent (bound-and-true-p sh-basic-offset)))
+                         (list "--indent-size" (number-to-string indent)))
+                       (when indent-tabs-mode "--tab")
+                       "-"))
+            apheleia-formatters)
+      ```
+
+      This adds an entry to `apheleia-formatters` for the `beautysh`
+      formatter. The evaluated entries makes it so that the `--tab`
+      flag is only passed to `beautysh` when the value of
+      `indent-tabs-mode` is true. Similarly the indent-size flag is
+      passed the exact value of the `sh-basic-offset` variable
+      only when it is bound. Observe that one of these evaluations
+      returns a list of flags whereas the other returns a single
+      string. These are substituted into the command as you'd expect.
 * `apheleia-mode-alist`: Alist mapping major modes and filename
   regexps to names of formatters to use in those modes and files. See
   the docstring for more information.
diff --git a/apheleia.el b/apheleia.el
index 5cc18db..625c44e 100644
--- a/apheleia.el
+++ b/apheleia.el
@@ -467,6 +467,26 @@ sequence unless it's first in the sequence"))
                                         (cl-return)))
                                   arg))
                               command)))
+      ;; Evaluate each element of arg that isn't a string and replace
+      ;; it with the evaluated value. The result of an evaluation should
+      ;; be a string or a list of strings. If the former its replaced as
+      ;; is. If the latter the contents of the list is substituted in
+      ;; place.
+      (setq command
+            (cl-loop
+             for arg in command
+             with val = nil
+             do (setq val (if (stringp arg)
+                              arg
+                            (eval arg)))
+             if val
+               if (and (consp val)
+                       (cl-every #'stringp val))
+                 append val
+               else if (stringp val)
+                 collect val
+               else do (error "Result of command evaluation must be a string \
+or list of strings: %S" arg)))
       `(,input-fname ,output-fname ,stdin ,@command))))
 
 (defun apheleia--run-formatters (commands buffer callback &optional stdin)