[erlang-questions] Indentation of multiline strings

Bengt Kleberg <>
Tue Feb 11 07:28:56 CET 2014


My comment that indention creating bugs "is only a problem with a
standalone formatter is probably based upon ..." was based upon this
list only mentioning standalone formatter when warning about bugs
created by indention.

Formatting imperfect inputs is is probably not a place where bugs
introduced by the formatter is a problem. That code will be tested.


On Tue, 2014-02-11 at 13:34 +1300, Richard A. O'Keefe wrote:
> 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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