"Prelude.read: no parse" when giving -M with --template

[K4zuki]

condition

• pandoc = 2.5
• OS = Ubuntu 16.04
• blank tex template = markdown/template.tex
• simple markdown file = markdown/markdown.md

This is a pen

how to reproduce

• input:
  pandoc -f markdown -t latex -M meta='1e' --template markdown/template.tex markdown/markdown.md

• output:
  pandoc: Prelude.read: no parse

• changing to meta='' stops error

• meta='1e|3e|5e|7e|9e' causes error but 'e' does not

• removing quote does not stop error

• position of -M meta=1e does not stop error

• similar with Error when html constains <ol start=''> #5162 ?

[jgm]

Simple repro:

% echo "" | pandoc  -M foo=1e -s
pandoc: Prelude.read: no parse

[jgm]

I confirm that this issue affects current HEAD (5101f43).

[mb21]

Interestingly even:

echo '' | pandoc -- -M foo=1e -s -f html -t native
pandoc: Prelude.read: no parse

The e looks suspiciously like something is trying to parse this as an exponential number, scientific notation. But what/where?

[mb21]

Reading to MetaInlines works:

cat foo.yaml
foo: "1e"

$ echo '' | stack exec pandoc -- --metadata-file foo.yaml -s -t native
Pandoc (Meta {unMeta = fromList [("foo",MetaInlines [Str "1e"])]})
[]

But -M would be reading to MetaString... possibly it's the Read instance on Many or something similar?

[mb21]

Stack trace:

$ echo '' | stack exec pandoc -- -M foo=1e -s -t native +RTS -xc
*** Exception (reporting due to +RTS -xc): (THUNK), stack trace: 
  Text.Pandoc.Class.liftIO,
  called from Text.Pandoc.Class.liftIOError,
  called from Text.Pandoc.Class.fileExists,
  called from Text.Pandoc.Class.readDataFile,
  called from Text.Pandoc.App.convertWithOpts,
  called from Main.main
*** Exception (reporting due to +RTS -xc): (THUNK), stack trace: 
  Text.Pandoc.Class.liftIO,
  called from Text.Pandoc.Class.liftIOError,
  called from Text.Pandoc.Class.fileExists,
  called from Text.Pandoc.Class.readDataFile,
  called from Text.Pandoc.App.convertWithOpts,
  called from Main.main
*** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace: 
  Text.Pandoc.Class.liftIO,
  called from Text.Pandoc.Class.liftIOError,
  called from Text.Pandoc.Class.fileExists,
  called from Text.Pandoc.Class.readDataFile,
  called from Text.Pandoc.App.convertWithOpts,
  called from Main.main
*** Exception (reporting due to +RTS -xc): (THUNK_1_0), stack trace: 
  Data.YAML.Schema.CAF:lvl130_r1kBj
  --> evaluated by: Data.YAML.Schema.coreDecodeFloat.float,
  called from Data.YAML.Schema.coreDecodeFloat,
  called from Data.YAML.Schema.coreSchemaResolver.schemaResolverScalar.go,
  called from Data.YAML.Schema.scalarTag,
  called from Data.YAML.Schema.coreSchemaResolver.schemaResolverScalar,
  called from Data.YAML.Schema.coreSchemaResolver,
  called from Data.YAML.decodeNode'.failsafeLoader.\,
  called from Data.YAML.decodeNode'.failsafeLoader,
  called from Data.YAML.Loader.decodeLoader.goNode,
  called from Data.YAML.Loader.decodeLoader.goDoc,
  called from Data.YAML.Loader.>>=,
  called from Data.YAML.Loader.manyUnless,
  called from Data.YAML.Loader.decodeLoader.goStream,
  called from Data.YAML.Loader.runParserT,
  called from Data.YAML.Loader.decodeLoader,
  called from Data.YAML.decodeNode',
  called from Data.YAML.decodeNode,
  called from Data.YAML.decode,
  called from Data.YAML.decodeStrict,
  called from Text.Pandoc.App.readMetaValue,
  called from Text.Pandoc.App.addMeta.v',
  called from Text.Pandoc.App.addMeta,
  called from Text.Pandoc.App.addMetadata,
  called from Text.Pandoc.Class.runIO,
  called from Text.Pandoc.Class.runIOorExplode,
  called from Text.Pandoc.App.convertWithOpts.runIO',
  called from Text.Pandoc.App.convertWithOpts,
  called from Main.main
*** Exception (reporting due to +RTS -xc): (base:GHC.Exception.SomeException), stack trace: 
  Data.YAML.Schema.CAF:lvl130_r1kBj
pandoc: Prelude.read: no parse
CallStack (from -prof):
  Data.YAML.Schema.CAF:lvl130_r1kBj (<no location info>)

[mb21]

Filed an upstream issue: haskell-hvr/HsYAML#7

[jgm]

@mb21 - Thanks for tracking this down!

[jgm]

Interestingly we don't get the error if you include this metadata value in the YAML metadata in a markdown file. Only with -M.

[jgm]

OK, we should be able to fix this in pandoc by using the failsafe schema instead of the json schema, which is used by default with decodeStrict, in Text.Pandoc.App's function readMetaValue.

[jgm]

Note also this workaround:

echo '' | pandoc -M foo='!!text 1e' -s