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

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/D... On Sat, Feb 20, 2016 at 11:59 AM, Benjamin Edwards 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 wrote:

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Yes, that is my point. Existentials cannot be unwrapped.

On Sat, Feb 20, 2016 at 4:18 AM, Kosyrev Serge < _deepfire@feelingofgreen.ru> wrote:

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Jeffrey Brown writes:

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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@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Jeffrey Benjamin Brown