[Haskell-cafe] What's this pattern called?

[Sean Leather]

>  I've heard fixed-point view, open datatypes and some others, but I'm
>> curious where this pattern comes up in literature and what it is called
>> there.
>>
>
> Tim Sheard and Emir Pasalic call this technique "two-level types" in their
> JFP'04 paper Two-Level Types and Parameterized Modules:
>
>    http://homepage.mac.com/pasalic/p2/papers/JfpPearl.pdf
>

Apparently from reading Section 2 of that paper, they would call ExprF
non-recursive type the structure operator. I think, by itself, the type
ExprF doesn't mean much. It really matters how it's used to determine what
you call it.

Martijn already mentioned the fixed-point view, but that only makes since in
the context of something like Fix:

> data ExprF r  = Add r r | Sub r r | Mul r r | Div r r | Num Int
> newtype Fix f = In (f (Fix f))
> type Expr     = Fix ExprF

I believe the F suffix that Martijn used typically means functor, and I
sometimes call types like this functors (with an optional but obvious
instance of Functor).

Regards,
Sean
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