Support conditionals

[fsteeg]

For a Fix like:

if equals(html.head.meta.property, 'description')
    map(html.head.meta.content, 'description')

With input like:

<meta property="description" content="Das Bewusstsein und..."/>

See also links in conditionals bullet point at https://github.com/metafacture/metafacture-fix#state

See also metafacture/metafacture-core#312

[fsteeg]

Input data samples:

https://www.hoou.de/materials/tutorial-lernen-lernen
(html.head.meta.content, html.head.meta.name, link.rel)

https://duepublico2.uni-due.de/servlets/OAIDataProvider?verb=ListRecords&metadataPrefix=mods
(metadata.mods.location.url)

[fsteeg]

The original sample above probably needs to be combined with a new list bind, like:

do list(path: html.head.meta.*, var: meta)
  if equals(meta.property, 'description')
    map(meta.content, description)
  end
end

See https://github.com/LibreCat/Catmandu/wiki/Binds

[fsteeg]

While we need support for conditionals in general, the examples here might be approached in a different way, resulting in more readable mapping code. See metafacture/metafacture-core#350

[fsteeg]

Original use case from first comment was solved with metafacture/metafacture-core#350, basic if statements implemented in #51, closing.