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

Clever! That answers my first question, but still leaves me unmotivated. Would GADTs allow me to offload some kind of work onto the compiler that I currently have to do myself? On Sat, Feb 20, 2016 at 2:00 PM, Matt wrote:

The pattern I've seen is:

data Some f where Some :: f a -> Some f

type G = Gr (Some Box') String

Ordinarily you lose the information about the `a`, but when you have a GADT, that allows you to recover type information. So you can match on:

f :: Some Box' -> String f (Some (Bi i)) = show (i + 1) f (Some (Bs s)) = s

Matt Parsons

On Sat, Feb 20, 2016 at 4:16 PM, Jeffrey Brown wrote:

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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 < edwards.benj@gmail.com> wrote:

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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

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-- Jeffrey Benjamin Brown

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