[Haskell-cafe] Managing a sum type with a lot of constructors

[Adam Flott]

My input I'm parsing into a data structure is URL query string like
<key>=<value> and separated by a delimiter. Ultimately what I want back is of
type `Data.Set.Set Text (v :: *)`. However I don't know how to represent that
and don't want to build an HList like interface for it.


On Wed, 23 Aug 2017 18:45:13 -0700
Michael Burge <michaelburge at pobox.com> wrote:

> If you have that many constructors, you probably aren't pattern-matching
> against the whole thing everywhere. So you could extract the few functions
> that use the entire type for case analysis into a typeclass, make each
> constructor its own type, and implement the typeclass:
> 
> Instead of:
> 
> something :: A -> IO ()
> something A1 = putStrLn "hello"
> something (A2 _) = putStrLn "world"
> 
> Use:
> 
> data A1
> data A2 = A2 Int
> 
> class RelatedConst a of
>   something :: a -> IO ()
> 
> instance RelatedConst A1 where
>   something _ = putStrLn "hello"
> 
> instance RelatedConst A2 where
>   something _ = putStrLn "world"
> 
> Then, each declaration and instance could go in its own file.
> 
> If on the other hand, you are using lots of partial case matches
> everywhere, see if there are commonalities and extract a typeclass for each
> group
> 
> 
> On Wed, Aug 23, 2017 at 6:29 PM, Adam Flott <adam at adamflott.com> wrote:
> 
> > I have a sum type with a lot of constructors and I'm not sure how to
> > represent
> > the type with maintainability in mind. For example,
> >
> >     data A = A1
> >            | A2 Int
> >            | A3 Text Int32 Bool
> >            | ...
> >            | A100 Bool
> >
> > Every inner type is concrete. There are 100+ constructors with no sign of
> > ever
> > getting reduced.
> >
> > What technique would you recommend to keep the sum type approach but not
> > having
> > to define them all in one spot? I'm thinking 1 inner type + 1 function to
> > construct per file (if that's possible).
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