Haskell syntax is great except for the indentation/layout rules. There is no simple definition of the grammar with regards to the layout. It's only specified as a headache-inducting operation of a state machine [1], and the implementation is quite hard to read. This is fine for writing Haskell, but quite painful if you want to parse it.

[1]https://www.haskell.org/onlinereport/haskell2010/haskellch10...