At least that agrees with what some Haskell compilers implement. No current Haskell compiler/interpreter agrees with what the report seems to say, that is that "A..." should be lexed as the two tokens "A.." and ".", and similarly, "A.where" should be lexed as "A.wher" followed by "e".A... should be split into "A.." and "."
I found a compromise: let's make it a lexing error! :-)
It seems that the source of the problem is the use of the (nonstandard) difference operator r1<r2> in the specification of the lexical syntax in the Haskell report [1]. It was presumably fairly innocent and easy to understand originally, but I guess that nobody understood its consequences when qualified names were introduced.
Anyway, this should teach us not to use homemade pseudo formal notation when defining a new language, but to stick to well-established, well-understood, tool-supported formalisms...
For the Programatica Haskell front-end, I have now switched to a regular expression compiler that has direct support for the difference operator, so hopefully, our implementation agrees with what the report specifies. (This is not necessarily a good thing, though, since it makes our front-end different from all other Haskell implementations :-)
For what it is worth, I tested "A...", "A.where" and "A.--" in the main Haskell implementations and in the Programatica Haskell front-end. The input was two modules A and B:
1 module A where 2 3 wher = id 4 e = id
1 module B where 2 import A 3 4 x = (A.where) 5 y = x
Here is the result:
GHC: B.hs:4: parse error on input `where' HBC: "B.hs", line 4, Bad qualified name on input:<eof> Hugs: ERROR "B.hs":4 - Undefined qualified variable "A.where" NHC98: Identifier A.where used at 4:6 is not defined. PFE: ok
If line 4 in module B is replaced with x = (A...):
GHC: B.hs:4: Variable not in scope: `A...' HBC: "B.hs", line 4, Bad qualified name on input:<eof> Hugs: ERROR "B.hs":4 - Undefined qualified variable "A..." NHC98: Identifier A... used at 4:6 is not defined. PFE: B.hs:4,13, before ): syntax error (A.. is lexed as A.. .)
If line 4 in module B is replaced with x = (A.--)
GHC: B.hs:4: Variable not in scope: `A.--' HBC: "B.hs", line 5, syntax error on input:= (treats -- as the start of a comment) Hugs: ERROR "B.hs":4 - Undefined qualified variable "A.--" NHC98: Identifier A.-- used at 4:6 is not defined. PFE: B.hs:5,1, before : syntax error (A.-- is lexed as A.- -)
I used the following versions
GHC 6.0.1 HBC 0.9999.5b Hugs 98 November 2002 NHC98 1.16 PFE 030912
-- Thomas H
[1] http://www.haskell.org/onlinereport/syntax-iso.html
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