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

12 Nov 2014

      I am imagining an alternative idiom for heterogeneous treatment in which
there is a single constructor, but the data type includes an attribute that
is a dictionary keyed with function names, leading to values that are the
corresponding code.

It seems like that would have to be memory-wasteful, duplicating the code
in every object, since Haskell does not permit pointers to a single code
source, but maybe I'm wrong about that.

On Wed, Nov 12, 2014 at 12:56 AM, Frank Staals  wrote:
...
Kannan Goundan  writes:
...
Karl Voelker  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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Re: [Haskell-cafe] Representing record subtypes, sort of.

Jeffrey Brown