Not sure if that's what you need:
data NodeF f = Node {name :: String, refs :: [f (NodeF f)]}
newtype Const a b = Const a
newtype Id a = Id a
type NodeS = NodeF (Const String)
type Node = NodeF Id
Отправлено с iPhone
Dec 12, 2010, в 20:54, Florian Weimer
Suppose I've got some named objects which reference other objects by name:
data NodeS = NodeS {nameS :: String, refsS :: [String]}
Through name resolution, the strings are translated to the actual nodes they denote:
data Node = Node {name :: String, refs :: [Node]} resolve :: [NodeS] -> Map String Node
NodeS and Node are quite similar, so they should probably be the same parametrized type. However, if I turn the type of the references into a type parameter, I need the type-level Y combinator to avoid an infinite type, and manipulating nodes turns a bit more tedious because of increased syntactic overhead.
Is there a third choice, beyond manually expanding the type definition or using Y?
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