After further study I believe existentials are not (at least alone) enough
to solve the problem.
They do allow a heterogeneous graph type to be defined and populated:
:set -XExistentialQuantification
import Data.Graph.Inductive
import Data.Maybe as Maybe
data ShowBox = forall s. Show s => SB s
instance Show ShowBox where show (SB x) = "SB: " ++ show x
type ExQuantGraph = Gr ShowBox String
let g = insNode (0, SB 1)
$ insNode (1, SB 'a')
$ empty :: ExQuantGraph
And once you've loaded those ShowBoxes, you can retrieve them:
getSB :: ExQuantGraph -> Node -> ShowBox
getSB g n = Maybe.fromJust $ lab g n
But you can't unwrap them. The following:
getInt :: ShowBox -> Int
getInt (SB i) = i
will not compile, because it cannot infer that i is an Int:
todo/existentials.hs:19:21:
Couldn't match expected type ‘Int’ with actual type ‘s’
‘s’ is a rigid type variable bound by
a pattern with constructor
SB :: forall s. Show s => s -> ShowBox,
in an equation for ‘getInt’
at todo/existentials.hs:19:13
Relevant bindings include
i :: s (bound at todo/existentials.hs:19:16)
In the expression: i
In an equation for ‘getInt’: getInt (SB i) = i
Failed, modules loaded: none.
On Fri, Feb 19, 2016 at 2:12 PM, Gesh
On February 19, 2016 10:54:11 PM GMT+02:00, Francesco Ariis < fa-ml@ariis.it> wrote:
hey Gesh,
you are right (not able to compile it atm too, but it looks correct and way elegant). Maybe post it in the Ml to help OP?
ciao ciao F
On Fri, Feb 19, 2016 at 04:59:56PM +0200, Gesh wrote:
I'm away from compiler at the moment, but... Shouldn't this work?
{-# LANGUAGE GADTs #-} data NodeS = HamsterS | PersonS data NodeP a where Hamster :: String -> NodeP HamsterS Person :: String -> NodeP PersonS data Node = forall a. NodeP a type Graph = Gr Node... hamsters :: NodeP PersonS -> ...
Basically the idea of that you reify the choice of constructor to the type level, permitting static restriction of the constructors used.
HTH, Gesh
Oops, meant to send to list. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
-- Jeffrey Benjamin Brown