Read Only Graph Builder In Functional Language

:: ROGraph a = ROGraph a [ROGraph a]
:: RODGraph a = RODGraph a [RODGraph a] [RODGraph a]

buildROGraph :: (a b (c,[b])) -> Map b (ROGraph c) | Lookup MapOnList b & UpdateDefault MapOnList b & Pairs a
buildROGraph m = buildGraph m \ id val fws bws = ROGraph val fws
buildRODGraph m = buildGraph m \ id val fws bws = RODGraph val fws bws

buildGraph rawGraph constructor = futureIndex
where
    raw = pairs rawGraph
    forwards = buildFastMap $ flip map raw \ (id,(a,fws)) = (id,fws)
    backwards = buildBackwards forwards
    futureIndex = buildFastMap $ flip map raw \ (id,(a,fws)) = (id,build id a fws)
    fromFuture ids = map (fromJustXXX o lookup futureIndex) ids
    build nodeId contents forwards
        # backwards = case lookup backwards nodeId of Just x -> x ; _ -> []
        = constructor nodeId contents (fromFuture forwards) (fromFuture backwards)

buildBackwards :: (a b [c]) -> a c [b] | Empty (a c [b]) & Pairs a & UpdateDefault a c
buildBackwards graph = seqMap processNode (pairs graph) empty
where
    processNode (id,forwards) = seqMapLFA forwards \ fr m = snd $ updateDefault fr (cons id) [] m

testGraph :: Map Int (Int,[Int])
testGraph = fromPairs [ @1 [2,3,4], @2 [4,1], @3 [1,2], @4 [3,4] ]
where
    @ a b = (a,(a,b))

Start = printRODGraph $ fromJust $ lookup (buildRODGraph testGraph) 1