Off-list, Greg Fitzgerald pointed out that:
Those wanting to use the SYB library may want to write something
like this for every datatype in their heirarchy:
deriving Data, Typable for Person, Team, Department, Company
That seems like a useful extension to me, and the meaning is obvious.
This would as far as I can tell not be possible (or at least not
readable) without the "for".
/Björn
On 5 okt 2006, at 20.58, Bjorn Bringert wrote:
We considered that syntax, but decided against it. Stand-alone
deriving declarations are made to be as similar as possible to the
current deriving mechanism, rather than be similar to instance
declarations. The basic reason for maintaining a syntactic
distinction between instance declarations and deriving declarations
is to make the programmer aware of the restrictions of the deriving
mechanism.
These are some things that make deriving declarations different
from instance declarations:
- You can only derive instances for data types and newtypes.
- For deriving declarations, the compiler figures out the
constraints, whereas the programmer writes them for instance
declarations.
- In GHC, you can declare non-Haskell98 instances such as Eq (C X)
where X is a concrete type, but you can't do deriving for them.
- When deriving instances of multi-parameter type classes (again
non-standard), the newtype for which the deriving is made must be
the last argument to the class. If the syntax were "deriving (Class
T1 ... Tn)", it might not be clear to the reader what type the
deriving is for.
I can't see any technical reason not to do as you propose. One
advantage would be that it makes it possible to fully subsume GHC's
current deriving extensions (though there are other ways to do
this, see my recent e-mail to ghc-cvs). One slight disadvantage is
that it does require a bit more footwork in the compiler to figure
out which type to do the deriving for.
/Björn
On 5 okt 2006, at 19.58, Iavor Diatchki wrote:
Hello,
A question about the syntax: would there be a problem if we made the
'deriving' declaration look like an instance? Then we would not need
the special identifier 'for', and also we have a more standard
looking
notation. I am thinking something like:
deriving Show SomeType
deriving Eq (AnotherType a)
-Iavor
On 10/5/06, Björn Bringert <[EMAIL PROTECTED]> wrote:
Simon Peyton-Jones wrote:
> | What is not so nice is that you take a new keyword ('for'),
which is
> | quite likely to have been used as a variable name in existing
code.
> (Or
> | does it work out to use one of the 'special' names here?)
>
> The latter is what Bjorn has done. That is, 'for' is only
special in
> this one context. You can use it freely otherwise. As I
understand it
> anyway.
Yes. There is even a "for" function somewhere in the libraries
(or was
it the testsuite, can't remeber), which tripped up one of my early
versions, before I had remembered to make "for" as a special ID
in the
parser.
> | I think it would be useful to write the proposal in complete
detail up
> | on the Haskell' wiki.
>
> Yes please. Bjorn? (It may just be a qn of transcribing the user
> manual stuff you have written.)
Sure. It seems that I have to be on the committee to write to the
Wiki.
Can I join it?
/Björn
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