This module provides a FieldGrammarParser, one way to parse .cabal -like files.

Fields can be specified multiple times in the .cabal files. The order of such entries is important, but the mutual ordering of different fields is not.Also conditional sections are considered after non-conditional data. The example of this silent-commutation quirk is the fact that

buildable: True
if os(linux)
  buildable: False

and

if os(linux)
  buildable: False
buildable: True

behave the same! This is the limitation of GeneralPackageDescription structure.

So we transform the list of fields [Field ann] into a map of grouped ordinary fields and a list of lists of sections: Fields ann = Map FieldName [NamelessField ann] and [[Section ann]].

We need list of list of sections, because we need to distinguish situations where there are fields in between. For example

if flag(bytestring-lt-0_10_4)
  build-depends: bytestring < 0.10.4

default-language: Haskell2020

else
  build-depends: bytestring >= 0.10.4

is obviously invalid specification.

We can parse Fields like we parse aeson objects, yet we use slightly higher-level API, so we can process unspecified fields, to report unknown fields and save custom x-fields.