Indeed! I think it'd be good to allow type signatures, including instance
signatures, in export lists
module Foo(
data T (f :: * -> *),
instance Functor f => Eq (T f),
g :: T f -> T f
)
The first step is to evolve a well-worked-out design. I think that'd be a very
valuable thing for someone to do. Indeed, I'd really like to see it in
Haskell', but it doesn't meet the "tried and tested" criterion.
I'm a bit reluctant to invest effort in half-way-house solutions, though.
Simon
| -Original Message-
| From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
| Behalf Of Serge D. Mechveliani
| Sent: 22 April 2008 13:33
| To: [EMAIL PROTECTED]
| Subject: instance export decls
|
| Dear GHC developers, people,
|
| Do you agree that there exists such a problem for a programmer as
| recalling exported instances?
| Have Haskell and/or GHC some constructs and tools to help the programmer
| to recall the exported instances for a module?
| Could GHC support the instance export messages for each module?
|
| In Haskell-98, the exported instances cannot be named explicitly in the
| export list.
| Right?
| But for easier understanding of a program, it is desirable to allow to
| name (in a short form) instances in the export list.
| For this reason, I add comments, and write the export like this:
|
| module Poly
| (WithHead(..), WithTail(..), -- classes
|Mon(..), Polynomial(..),
|lc, polMons
|-- , instances
|-- for Mon: WithHead, List;
|-- for Polynomial: WithHead, Cast Polynomial Mon
| )
| The comment of kind "-- , instances ..."
| helps to recall which instances are exported, without inspecting all
| the (lengthy) module source.
|
| But when the programmer changes the source, one often forgets to
| add/remove the needed comments about instances.
| I often forget them (maybe, lazy to recall) when I move pieces of code
| between modules.
|
| So, I suggest the following feature for GHC.
|
| 1. To allow the export declarations of kind instance {}
|(maybe to suggest this for Haskell ?)
| 2. If the module under compilation has the word `instance' in its
|export list,
|then ghc finds the difference set diff for the exported instance
|kinds eKinds and the instance kinds nKinds named in the export
|list. If not $ null diff, it issues the message:
|"Warning: the exported instance kinds and the instance kinds named
| in the export differ in the following items: ...".
|
| Seeing such a message, the user corrects the export list in the source
| according to diff.
|
| For the export list and for their messages, it is probably better to
| use a short denotation: the instance kind rather than full instance
| declaration. In the instance kind, the part of "(...) =>" is skipped.
|
| What people think of this suggestion?
|
| Thank you in advance for your notes and help,
|
| -
| Serge Mechveliani
| [EMAIL PROTECTED]
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