I'll push back and say that the lexer/parser split is well worth it.

And the best thing about the parser combinator approach is that each is just a kind of parser, something like

  type Lexer = ParsecT e ByteString m [Token]

  type Parser = ParsecT e [Token] Expr

All the usual helper functions like many or sepBy work equally well in the lexing and parsing phases.

It really beats getting to the parentheses-interacting-with-ordering-of-division-operations stage and still having to think "have I already trimmed off the whitespace here or not?"