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

[Frank Staals]

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