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: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On | Behalf Of Serge D. Mechveliani | Sent: 22 April 2008 13:33 | To: glasgow-haskell-users@haskell.org | 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 {<type>} | (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 | mechvel@botik.ru