[Haskell-cafe] Selective deriving
ruihfazevedo at gmail.com
Mon Feb 20 04:51:16 UTC 2017
I don't know if there is some extension that already does this, or any
other method of doing it.
My search on deriving ghc mechanisms and extensions gave me no
the point is to have some "help" on derivation or some sort of derivation.
actual derivation schema seems to require that all members (constructor
arguments) instantiate the deriving class. This is the way of doing it
for classes like Eq, works for Show and Ord but eventually it is a very
strong requirement even for Eq sometimes. Of course I can do a manual
derivation to state something else, and that sometimes can be
frustrating because the derivations are way trivial and susceptible of
There is no fancy things added, its all standard haskell, user could
laboriously type this.
As an alternate derivation mechanism i seek selective deriving, like this:
class Named o where name::o->String
data A = A String deriving (Show)
instance Named A where name="Object"
till now just normal derivation
data B = B A Int deriving (Show) extending (Named)
the mechanism on extending requires that one and only one of the type
constructor's direct arguments instantiates the class.
this avoids the C++ diamond inheritance problem, user can recur on
this will generate the code:
instance Named B where name (B a _)=name a
alternative can be: extending (Named A,...)
this latter form is rater interesting as it cuts down the implementation
and still avoid the diamond problem even if multiple members instantiate
the same class
Are already any extension doing this?
Can i achieve the same result by other forms?
What is the best way to implement this thing? plug-ins seem not to be
able to change syntax (right?)
I've also looked at -F -pgmF options to use a preprocessor... can a
preprocessor insert code to reference the original line numbers, just
because messing with line numbers will make this unusable.
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