[Haskell-cafe] A backwards-compatible record proposal
brianh at metamilk.com
Sun Aug 20 06:54:07 EDT 2006
Bulat Ziganshin wrote:
> Hello Brian,
> Saturday, August 19, 2006, 12:21:34 PM, you wrote:
>> ie putting a '.' before each field name. The intended meaning is
>> that dotted field names do *not* generate top level functions.
>> Instead they allow the compiler to generate instance decls as
>> follows, where we've introduced a new form of identifier, the dotted
>> id, which behaves as a postfix operator which binds more tightly
>> than function application and can also be used as a class name (by
>> the compiler only):
>> class (.x) :: a b | a -> b where
>> (.x) :: a -> b
> this means that foo.bar should be parsed differently by _lexer_
> depending on is there any .bar field available in current or any
> imported module
I just assumed that foo.bar would always be lexed as "foo" ".bar" so if
composition was intended there would need to be a space between the "." and
the "bar", but this breaks backwards compatibility (I hadn't thought of this
when I made the proposal because I assumed everyone always writes "f . g"
and not "f.g" when they mean composition but of course the latter is
currently parsed as composition also).
> How about omitting '.' and using '#' operation for record access? the
> only problem will that '#' should have larger priority than ' '. i.e.
> function application. I had proposal on it, but it's too daring -
> raise priority of operations when they are written without spaces
> around, i.e.
> object#call x+y t#field z*2+1
> treated as
> (object#call) (x+y) (t#field) (z*2+1)
Intuitively this makes sense, because you'd expect no spaces to mean "glue
this together as tight as possible", but on the other hand I personally find
the second version with brackets is visually much easier to read even though
it's a bit more inconvenient to write.
1) The first version is not backwards compatible either.
2) object#call should not be treated as (object # call) because it means
((#call) object) so whatever symbol is used for field selection, it needs to
be glued onto the field name by the lexer not the parser.
Also I was hoping to be able to use the "." so that field access would
follow the C, C#, Java conventions and because # is used by ghc to mean
"unboxed" so the following might be confusing:
object#call 1# x#y # k (#g,a#)
object.call 1# x.y # k (#g,a#)
((object.call) 1# (x.y)) # (k (# g, a #))
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.
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