[Haskell-cafe] Manual type-checking in graphs: Avoidable?

[Jeffrey Brown]

Interesting! I have two questions.

(1) Given that Graph is of kind * -> * -> *, rather than (* -> *) -> * ->
*, how can I use a GADT? The first graph using existentials defined earlier
in this thread looked like:

    data Box = forall s. Show s => Box s
    type ExQuantGraph = Gr Box String

If instead I use a GADT:

    data Box' a where
      Bi :: Int -> Box' Int
      Bs :: String -> Box' String

then I can't define a graph on

    type G = Gr Box' String

because Box is not a concrete type. I could specify (Box a) for some a, but
then I lose the polymorphism that was the purpose of the GADT.

(2) Would a GADT be better than what I'm already doing? Currently I define
a Mindmap[1] as a graph where the nodes are a wrapper type called Expr
("expression"):

    type Mindmap = Gr Expr _ -- the edge type is irrelevant
    data Expr = Str String | Fl Float
              | Tplt [String] | Rel | Coll
              | RelSpecExpr RelVarSpec deriving(Show,Read,Eq,Ord)

I do a lot of pattern matching on those constructors. If I used a GADT I
would still be pattern matching on constructors. So do GADTs offer some
advantage?

[1]
https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/src/Dwt/Graph.hs

On Sat, Feb 20, 2016 at 11:59 AM, Benjamin Edwards <edwards.benj at gmail.com>
wrote:

> if you are willing to have a closed universe, you can pattern match on a
> gadt to do do the unpacking
>
> On Sat, 20 Feb 2016 at 19:19 Jeffrey Brown <jeffbrown.the at gmail.com>
> wrote:
>
>> Yes, that is my point. Existentials cannot be unwrapped.
>>
>> On Sat, Feb 20, 2016 at 4:18 AM, Kosyrev Serge <
>> _deepfire at feelingofgreen.ru> wrote:
>>
>>> Jeffrey Brown <jeffbrown.the at gmail.com> writes:
>>> > After further study I believe existentials are not (at least alone)
>>> > enough to solve the problem.
>>> ..
>>> > getInt :: ShowBox -> Int
>>> > getInt (SB i) = i
>>> >
>>> > will not compile, because it cannot infer that i is an Int:
>>>
>>> You take a value of an existentially quantified type (which means it
>>> can be anything at all, absent some extra context) and *proclaim* it
>>> is an integer.
>>>
>>> On what grounds should the compiler accept your optimistic restriction?
>>>
>>> --
>>> с уважениeм / respectfully,
>>> Косырев Сергей
>>>
>>
>>
>>
>> --
>> Jeffrey Benjamin Brown
>> _______________________________________________
>> Haskell-Cafe mailing list
>> Haskell-Cafe at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>
>


-- 
Jeffrey Benjamin Brown
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