I'd have thought it would have been simpler to just make the rule that -2 (no spaces between '-' and '2') would be a single lexeme
I'd have thought so too, until I implemented a parser with exponentiation. It is easy to get confused and make a parser that is too eager to include the minus sign as part of a numeric literal instead of as subtraction or unary negation (all you parser-with-exponentiation-implementers out there, pay attention!). And since many programming languages (specifically C) don't have syntax for exponentation as an infix operator (nothing authoritative to copy precedence from), I had to implement this myself, get confused and see that it was so---(I tried making the literal include the minus sign if there was no space). I never noticed this before because in a C-like language: -4*2 is the same whether parsed as (-4)*2 or -(4*2) but -4^2 is not the same whether parsed as (-4)^2 or -(4^2) (the correct version) Basically, before someone argues this with me, -4^2 should parse the same as - 4^2 which should be the same thing as 0 - 4^2 (you don't want -4^2 and 0-4^2 giving different results, no matter how much you think whitespace belongs around operators) Math follows these same rules but it's slightly harder to get confused because of the way exponentiation is written by superscripting. See http://mathforum.org/library/drmath/view/55709.html and http://mathforum.org/library/drmath/view/53240.html. I thought this was surprising, that parsing the minus sign into lexemes would cause such confusion, but it is born out in many places (Python, Frink (http://futureboy.homeip.net/frinkdocs/FrinkApplet.html), etc.) (Note: this email isn't about Haskell specifically and I'm sure issues with the minus sign in Haskell are more confusing than this; this is purely about parsing a C-like langauge extended with exponentionation and how including the minus sign in the literal is dangerous in conjuntion with exponentiation.) Jared. -- http://www.updike.org/~jared/ reverse ")-:"