The parser itself is unaware of indentation, and instead only parses explicit delimiters which are inserted by this layout algorithm (much like Haskell). This is convenient because the actual grammar can be specified apart from the indentation rules. Haskell has a few problematic productions which make it impossible to implement a purely lexical layout algorithm, so it also has an additional (and somewhat contentious) parser error side condition. PureScript does not have these problematic productions (particularly foo, bar :: SomeType syntax in declarations), but it does have a few gotchas of it's own. The algorithm is "non-trivial" to say the least, but it is implemented as a purely lexical delimiter parser on a token-by-token basis, which is highly convenient, since it can be replicated in any language or toolchain. There is likely room to simplify it, but there are some seemingly innocuous things that complicate it.

Naked commas (case, patterns, guards, fundeps) are a constant source of complexity, and indeed too much of this is what prevents Haskell from having such an algorithm. Unquoted properties for layout keywords introduce a domino effect of complexity since we have to mask and unmask any usage of . (also in foralls!) or labels in record literals.