Marc Weber wrote:
Trying to copy paste all the definitions which I think I need to make
the countFields sample from the tutorial work into my own file and
prefixing it by my_ yields this code, and the error shown at the bottom.
Which magic is happening in the standard library I've missed?
Also where exactly is the recursion (repeat until the attribute set has
been processed) happening?
Is it somewhere hidden in the compiler?
While [folder] is a type family that you can define yourself, folder
_inference_ is built into the compiler and only applies to
[Top.folder]. Your example doesn't specify a value of your own [folder]
type, but the compiler won't infer such a thing. The error message is a
good one, to tell you that the function is only partially applied and
isn't ready to yield an [int] yet. (A "real" [folder] argument is
implicit by default, but not so for your own type family.)
When the compiler synthesizes these values, they're built out of the
combinators in [Top.Folder]. "The recursion" is encoded in the
combinator tree, whose size varies with the size of the term to build a
folder for.
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