[erlang-questions] Indentation of multiline strings
Richard A. O'Keefe
ok@REDACTED
Tue Feb 11 01:34:05 CET 2014
It is now more years than I care to recall since I wrote
the essay "Delenda est preprocessor."
It is extremely hard for indent(1) to do a good job of C.
Which is why it doesn't, really.
For example, here is some legal C code.
if (border) printf("\\hline");
rflag = border;
for_each_element_child(e0, i, j, e1)
printf(rflag ? "\\\\\n" : "\n");
rflag = true;
cflag = false;
for_each_element_child(e1, k, l, e2)
if (cflag) printf(" & ");
cflag = true;
walk_paragraph("", e2, "");
end_each_element_child
end_each_element_child
if (border) printf("\\\\\\hline");
This is part of a program that turns slides marked up in SGML
into LaTeX. It outputs the body of a table.
for_each_element_child(Parent, I, J, Child)
<code for element children>
if_no_element_child
<code for no elements>
end_each_element_child
iterates over the children of Parent. The variable I
counts all children. The variable J counts those children
that are elements (as opposed to processing instructions,
comments, PCDATA, unresolved entity references, &c). Child
is bound to the corresponding child. Whenever such a Child
is for, <code for element children> is executed. If Parent
has no child that is an element, <code for no elements> is
executed; that part is optional.
- If an indenter formats this with no knowledge of preprocessing,
it will generate something unspeakably horrible.
- If an indenter is smart enough to look inside the macros, it
will produce something that makes sense, but since
- for_each_element_child has four left curly braces
- if_no_element_child has }} if (...) {{
- end_each_element child has four right curly braces
the result will still not be something you want to read.
- Only if an indenter can be told about *these* macros specifically
will anything reasonable happen.
- You really really REALLY do not want to do this stuff without
such macros. You don't want to see how
for_each_descendant(Ancestor, Descendant)
<recursion without recursion>
end_each_descendant
is implemented. You really don't.
In practice, this means that the tolerably large amount of code
I've written marching over SGML/XML documents -- and it may be
C, but it's *still* simpler than XSLT! -- *cannot* be indented
with indent(1) and still less can it be indented with emacs.
What is the relevance of this to Erlang?
Well, Erlang copied the C preprocessor, and must suffer the
consequences.
On 10/02/2014, at 8:01 PM, Bengt Kleberg wrote:
> Greetings,
>
> One reason against a standalone formatter is that it could introduce
> bugs in the code. That this is only a problem with a standalone
> formatter is probably based upon that nobody would use a editor based
> formatter and then not compile/test after wards.
I do not believe that it IS "only a problem with a standalone
formatter" and I for one always check the output of indent(1).
>
> To make a stand alone formatter with better level of confidence I
> suggest using Erlang/OTP tools to create one of the intermediate level
> formats (Core Erlang, "parse trees", the simplest/quickest/...) that
> removes formatting. If this is done before and after the standalone
> formatting, and the result is the same, then no bugs have been
> introduced.
I've always liked indenters that could cope with imperfect inputs.
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