# Parsing TeX output with Parsec

**URL:** <https://discourse.haskell.org/t/parsing-tex-output-with-parsec/6678>\
**Category:** Learn\
**Created:** [June 26, 2023, 7:42am UTC](https://discourse.haskell.org/t/parsing-tex-output-with-parsec/6678 "2023-06-26T07:42:56Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![indrjo](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/indrjo/32/3306_2.png) [@indrjo](https://discourse.haskell.org/u/indrjo)\
**Post date:** [June 26, 2023, 7:42am UTC](https://discourse.haskell.org/t/parsing-tex-output-with-parsec/6678/1 "2023-06-26T07:42:57Z")

</div>

I must say first of all I’m quite new in Parsec. I am trying to write a parser in order to catch errors during the creation of a document via TeX. I have already written something using pcre-light, but I think it might be nice to learn some Parsec. The book “Real World Haskell” seems quite outdated regarding this topic. I read something on Hackage, and here is my attempt.

```haskell
{- cabal:
    build-depends: base, parsec
-}

module TeXOutputsParser where

import Text.Parsec
import Text.Parsec.String

data TeXError = FileNotFound String
              | OtherTeXError String
  deriving (Show)

texError :: GenParser Char s TeXError
texError = do
  char '!' >> spaces
  try fileNotFound <|> otherTeXError <?> "tex-error-parser failed..."
  where
    fileNotFound :: GenParser Char s TeXError
    fileNotFound =
      FileNotFound <$>
        between (string "LaTeX Error: File `")
                (string "' not found")
                (many (noneOf "'"))
    otherTeXError :: GenParser Char s TeXError
    otherTeXError =
      OtherTeXError <$> many (noneOf "?")

texErrors :: GenParser Char s [TeXError]
--texErrors = many (noneOf "!") >> texError `sepBy` many (noneOf "!") 
texErrors = (eof >> return [])
        <|> (texError >>= \err -> (err:) <$> texErrors)
        <|> (many (noneOf "!") >> texErrors)

```

The question are:

1. As you can see, I have commented one LOC, being that one of my early attempts. But it fails, see the output below for example. Why?
2. Any suggestion? Can I make the code shorter and more “idomatic”?

Here is a sample output:

```haskell
This is LuaHBTeX, Version 1.17.0 (TeX Live 2023) 
 restricted system commands enabled.
(./main.tex
LaTeX2e <2023-06-01> patch level 1
L3 programming layer <2023-06-16>
 (~/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2023/05/17 v1.4n Standard LaTeX document class
(~/texlive/texmf-dist/tex/latex/base/size11.clo))
(~/texlive/texmf-dist/tex/latex/fontspec/fontspec.sty
(~/texlive/texmf-dist/tex/latex/l3packages/xparse/xparse.sty
(~/texlive/texmf-dist/tex/latex/l3kernel/expl3.sty
(~/texlive/texmf-dist/tex/latex/l3backend/l3backend-luatex.def)))
(~/texlive/texmf-dist/tex/latex/fontspec/fontspec-luatex.sty
(~/texlive/texmf-dist/tex/latex/base/fontenc.sty)
(~/texlive/texmf-dist/tex/latex/fontspec/fontspec.cfg)))
(~/texlive/texmf-dist/tex/latex/kantlipsum/kantlipsum.sty)

! LaTeX Error: File `not-a-package.sty' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: sty)

Enter file name: 
! LaTeX Error: File `not-a-package2.sty' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: sty)

Enter file name: (./main.aux) (~/texlive/texmf-dist/tex/latex/base/ts1cmr.fd)
[1{~/texlive/texmf-var/fonts/map/pdftex/updmap/pdftex.map}]
(./main.aux))
 406 words of node memory still in use:
   3 hlist, 1 vlist, 1 rule, 2 glue, 3 kern, 1 glyph, 4 attribute, 48 glue_spec
, 4 attribute_list, 1 write nodes
   avail lists: 2:82,3:20,4:5,5:82,6:8,7:2054,8:1,9:72,10:3,11:144
<~/texlive/texmf-dist/fonts/opentype/public/lm/lmroman10-regular.otf
><~/texlive/texmf-dist/fonts/opentype/public/lm/lmroman12-regular.ot
f><~/texlive/texmf-dist/fonts/opentype/public/lm/lmroman17-regular.o
tf>
Output written on main.pdf (1 page, 12828 bytes).
Transcript written on main.log.

```

Thank you for any help.

---

<div class="post-metadata">

**Author:** ![turion](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/turion/32/989_2.png) [@turion](https://discourse.haskell.org/u/turion)\
**Post date:** [June 26, 2023, 10:25am UTC](https://discourse.haskell.org/t/parsing-tex-output-with-parsec/6678/2 "2023-06-26T10:25:23Z")

</div>

Not entirely sure, but do you encode in your parse somewhere that the `!` must be at the beginning of a line? If not, I think that would improve it.

---

<div class="post-metadata">

**Author:** ![blamario](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/blamario/32/860_2.png) [@blamario](https://discourse.haskell.org/u/blamario)\
**Post date:** [June 26, 2023, 8:41pm UTC](https://discourse.haskell.org/t/parsing-tex-output-with-parsec/6678/3 "2023-06-26T20:41:56Z")

</div>

The combinator `sepBy` expects its input to end with its first argument. In your case, every separator `many (noneOf "!")` would have to be followed by `texError`. If you replace `sepBy` with `sepEndBy` your one-liner works.