Re: Type of the `filter' argument to traversal functions

[John MacFarlane <jgm-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org>]

Anton Shepelev <anton.txt-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:

> John MacFarlane to Anton Shepelev:
>
>> > The  documentation to walk_block() and walk_fil-
>> > ter() states that the second parameter is "a Lua
>> > filter (table of functions) to be applied within
>> > the block element".
>> > [...]
>> > Is the documentation incorrect or do I misinter-
>> > pret  it?  For  aught I know, {Span = OnSpan} is
>> > not a "table of functions".
>>
>> { } makes a table.  OnSpan is a function.   So,  a
>> table of functions.
>
> Please,  bear with me. A table in Lua is the one-to-
> rule-them-all complex type, that can represent  many
> sturctures,  including  array, associative array, or
> structure.  On the other hand,  the  generally  used
> meaning of `table' in programming and mathematics is
> a two-dimensional sturcture  with  columns  and  any
> number of rows from zero up, which, too, can be rep-
> resented as a table in Lua, although it would  be  a
> compound one.

I would have thought that in the context of documenting a Lua
function 'table' would be naturally interpreted as 'Lua table'.

> But  since the parameter is described as "a Lua fil-
> ter" and "a table of functions", the only way to in-
> terpret it is as describing the structure of the Lua
> filter itself, which is a table of functions in both
> Lua and general terminology:
>
>                 { { Id1 = Func1 }
>                   { Id2 = Func2 }
>                   { Id3 = Func3 } }

According to the docs, "Lua filters are tables with element names
as keys and values consisting of functions acting on those
elements." So yes, the parameter to walk_block is a Lua filter as
defined in the documentation.

If you had

  { { Id1 = Funct1,
      Id2 = Funct2 } }

that would be an array of Lua filters. But now, I see where you
may be getting confused. Reading on in the docs: "Pandoc expects
each Lua file to return a list of filters. The filters in that
list are called sequentially, each on the result of the previous
filter." It may be confusing that one often uses the term
"lua filter" for the file itself. But the file can return
a list (technically an array, implemented as a Lua table) of
filters.  I suspect you are confusing an array of Lua filters --
which can be defined in a single Lua file -- with a Lua filter.