[Haskell-cafe] Representing record subtypes, sort of.

[Jeffrey Brown]

What is the tradeoff here? Would polymorphic containers prohibit the
compiler from the deep reasoning it can do without them?

On Wed, Nov 12, 2014 at 12:56 AM, Frank Staals <frank at fstaals.net> wrote:

> Kannan Goundan <kannan at cakoose.com> writes:
>
> > Karl Voelker <karl <at> karlv.net> writes:
> >
> >> {-# LANGUAGE DataKinds #-}
> >> {-# LANGUAGE GADTs #-}
> >> {-# LANGUAGE KindSignatures #-}
> >> module FS where
> >>
> >> type Date = String
> >>
> >> data FileKind = FILE | FOLDER
> >>
> >> data Entry (k :: FileKind) where
> >>   File   :: String -> Date -> Int -> Entry FILE
> >>   Folder :: String -> String -> Entry FOLDER
> >
> > This is a little beyond my Haskell knowledge.  What would the function
> > signatures look like?  Here are my guesses:
> >
> >     listFolder :: Path -> [Entry ?]
>
> Unfortunately, we cannot have our cake and eat it as well. Entry FILE
> and Entry FOLDER are now different types, and hence you cannot construct
> a list containing both. In other words; we cannot really fill in the ?
> in the type signature (or at least not that I'm aware of). Either we use
> Either (pun intended): listFolder :: Path -> [Either (Entry FILE) (Entry
> FOLDER)] or you have to create some existential type around an Entry
> again, i.e.
>
> data SomeEntry where
>   SomeEntry :: Entry k -> SomeEntry
>
> listFolder :: Path -> [SomeEntry]
>
> You can get the file kind back by pattern matching again.
>
> >     createFolder :: Path -> Entry FOLDER
> >     createFile :: Path -> Entry FOLDER
>
> the second one should produce something of type Entry FILE.
>
> > Also, lets say I wanted to just get the "id" fields from a list of
> `Entry`
> > values.  Can someone help me fill in the blanks here?
> >
> >     l :: [Entry ?]
> >     let ids = map (?) l
>
> This is basically the same issue as before. You cannot construct a list
> that contains both Entry FILE and Entry FOLDER values. We can use type
> classes together with the SomeEntry solution above though.
>
> ----
>
> In general I like the fact that we can use the GADTs to obtain extra
> type level guarantees. However, working with lists (or other data
> structures) with them is a crime. I think for that, we need better
> support for working with hetrogenious collections.
>
> --
>
> - Frank
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