Improve error messages about pandoc --to pdf

[Toxaris]

Pandoc can produce pdfs, but there is no pdf writer. Instead, in order to produce pdfs, you have to use the latex writer with an output file name that ends in .pdf. I understand the technical reasons for this behavior. Unfortunately, the user interface is not perfect if people accidently use a --to pdf flag:

`>echo hello | pandoc --to pdf
pandoc: Unknown writer: pdf

> echo hello | pandoc --to pdf -o hello.pdf
pandoc: cannot produce pdf output with pdf writer

The first error message suggests that pandoc cannot produce pdfs, and the second error message suggests that there is a pdf writer, but it is broken. Both error messages fail to help users achieve their goal, presumably to create a pdf with pandoc.

I see two options for how to improve the behavior:

1. These error messages could explain how to produce a pdf. Approximately:

   Unknown writer: pdf.
   To create a pdf with pandoc, use the latex writer and an output file name:
   pandoc --to latex -o filename.pdf
   

2. These invocations could try to produce a pdf, essentially "faking" a pdf writer. The first invocation would have to fail because there's no output file name:

   `>echo hello | pandoc --to pdf
   pandoc: Specify output file name for pdf with -o filename.pdf
   

   The second invocation would have the same as pandoc --to latex -o hello.pdf.

[KurtPfeifle]

If you want PDF output, you do not need any --to. You only need an output filename with a .pdf suffix: -o output.pdf. The only variable is that you may use a different latex engine: --latex-engine=xelatex or --latex-engine=lualatex. By default (if you do not name an alternative latex-engine), it will behave as if you used --latex-engine=pdflatex.

But I agree, the error message that pops up if you use --to=pdf can be improved and made more clear.

[Toxaris]

@KurtPfeifle: Thanks for the extra info. Can you propose short and helpful error messages for the two example invocations above?

[KurtPfeifle]

echo hello | pandoc --to pdf
  pandoc: Unknown writer: pdf
  pandoc: To create a PDF, specify output file with suffix '.pdf'. Example: '-o filename.pdf'.

[Toxaris]

Given @KurtPfeifle's error message, I would try next:

echo hello | pandoc --to pdf -o filename.pdf
  pandoc: cannot produce pdf output with pdf writer

[KurtPfeifle]

echo hello | pandoc --to pdf
  pandoc: Unknown writer: pdf
  pandoc: To create a PDF, do not use '--to pdf'. 
  pandoc: For PDF output, simply specify output file with suffix '.pdf'. 
  pandoc: Example: 'pandoc -o filename.pdf [...other options...]'.

[jgm]

@Toxaris - thanks for the suggestion.

[mariussoutier]

I guess this got lost somewhere, because right now the error message is pretty confusing: cannot produce pdf output from pdf

[jgm]

@mariusoutier What are you doing to get that message, and what version of pandoc are you using?

[mariussoutier]

pandoc --from markdown --to pdf --output gs.pdf produces the confusing error message. pandoc --from markdown --output gs.pdf works as expected.

[jgm]

OK, I see. This makes sense if you understand what's going on internally -- pandoc generates latex, html, or ms and converts that to a pdf -- but I agree it's confusing.