[Haskell-cafe] Avoiding the Y combinator for self-referencing types

[Miguel Mitrofanov]

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 <fw at deneb.enyo.de> написал(а):

> 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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