| Is the following legal Haskell?
|
| > infixr 0 `foo`
| > infixr 0 `bar`
| > ...
| > dubious a b c = a `foo` b `bar` c
|
| According to the grammar in the Haskell report, I don't think it is.
| However, ghc-0.24 (ancient, I know) and Hugs 1.3 both accept it without
| complaint.
I believe that it is legal, and attach an (inverted) parse tree below to
justify that claim:
b :: exp10 c :: exp10
---------- ----------
a :: exp10 . .
---------- . .
. --------- ---------
. b :: exp1 `bar` :: qop(r,0) c :: exp1
---------- ---------------------------------------
a :: exp1 `foo` :: qop(r,0) (b `bar` c) :: rexp0
----------------------------------------------------
(a `foo` (b `bar` c)) :: rexp0
------------------------------
(a `foo` (b `bar` c)) :: exp0
-----------------------------
(a `foo` (b `bar` c)) :: exp
However, in return, perhaps somebody can supply me with parse trees for
the following:
- - 1 (accepted by nhc and hbc)
(- 1 `n6` 1) where infix 6 `n6` (accepted by nhc, hbc, ghc)
(- 1 `r6` 1) where infixr 6 `r6` (accepted by nhc, hbc, ghc)
As I read the grammar (for 1.3), all of these should be illegal. In
fact, the compilers don't even give consistent answers for the last
example: ghc and nhc treat it as (-(1 `r6` 1)) while hbc treats it
as ((-1) `r6` 1).
The Hugs and Gofer parsers have always treated unary minus (incorrectly,
if you use the Haskell report as your bible) as having a higher
precedence than any operator, so Hugs and Gofer don't parse the above
correctly either. It was while I was modifying the next release of Hugs
to give the behaviour specified in the Haskell report that I noticed
these discrepancies. (Disclaimer: The above results were obtained with
the versions of each compiler that I have to hand, and not necessarily
the most recent versions.)
All the best,
Mark
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`Joy, Happiness, and Banana Mochas all round'
-- The JavaBeans(TM) 1.0 API specification
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