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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