Hi,

I think what Edward describes can be achieved by taking a multi index set library, and using only one index. I searched Hackage and found many unmaintained options:

  deprecated: tables, HiggsSet

  data-store -- needs dependency version bumps at least

These are probably usable options:

  ixset, ixset-typed, data-map-multikey

Based on my search, it seems to me that there are very few maintained libraries between Data.Map and packages like persistent and groundhog. Those two libraries provide convenient access to sql databases, which is probably unnecessarily complex for the original problem.

Regards,

Adam

On Fri, Jul 8, 2016 at 9:40 AM, Edward Z. Yang <ezyang@mit.edu> wrote:

Hello Lian,

I recently wrote a module for just this purpose.  Here is the approach
that I (and Edward Kmett) like to take:

1. Create a type class with an associated type representing elements
whic have keys:

    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE UndecidableInstances #-}
    class Ord (Key a) => HasKey a where
        type Key a :: *
        getKey :: a -> Key a

2. Write new data structures which utilize this type-class.

    import qualified Data.Map as OldMap
    data Map a = OldMap.Map (Key a) a
    insert :: HasKey a => a -> Map a -> Map a

  These structures are responsible for maintaining the key-value
  invariants (which can be tricky at times; be careful!)

There are other approaches too; for example you can use a multiparameter
type class with a functional dependency. "HasKey k a | a -> k"

Unfortunately I am not aware of any standardized naming scheme
for HasKey/getKey.

Edward

Excerpts from Lian Hung Hon's message of 2016-07-08 09:35:53 -0400:

> Dear cafe,
>
> What is the idiomatic way to "split" records into their natural keys and
> content in a data structure? For example, given a user:
>
> data User = { username :: ByteString, hash :: ByteString, address :: Text,
> ... }
>
> Using map, a first choice would be Map ByteString User, but this leads to
> duplication of the username. And it is possible to make mistakes, such as
>
> insert "John" (User "Jane" ...
>
> What does cafe think? Is there any pattern for this? This is probably just
> a small nit in the overall architecture, but I'm curious to know the clean
> way to do it.
>
>
> Regards,
> Hon

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