What is the reason of error "Not a data constructor"?

Question

I have got an error: Not a data constructor: "%:":

data KV = forall a. Show a => (%:) Text a

Interestingly, but :% is fine as data constructor! %% is not fine again. But %% is fine to be infix function. Why is it treating as an error? What is the difference between these variants?

But I get this error in `data KV = ...` definition, not in the place where I use it — RandomB, Jul 02 '19 at 14:41
I am using `Data.Aeson`, is it possible that it exports such functions/terms? I never met them before.. — RandomB, Jul 02 '19 at 14:43
Related question: https://stackoverflow.com/questions/30039123/haskell-why-arent-infix-type-constructors-allowed — AJF, Jul 02 '19 at 15:00
@AJFarmar Ooh, this - "The caveat is that data constructors (not type constructors) must start with a colon:". Seems this is the reason — RandomB, Jul 02 '19 at 15:02

score 4 · Accepted Answer · answered Jul 02 '19 at 15:02

4

It's not a valid name for a data constructor. All infix operator data constructors must start with :. So (:%) would be fine.

This is just the operator equivalent of "constructors must start with capital letters", as a syntactic means to distinguish constructors from other names when pattern matching.

answered Jul 02 '19 at 15:02

Carl

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I memorize this as "`:` counts as an uppercase character". – chi Jul 02 '19 at 15:23
I just remember that the non-empty list constructor `:` is the simplest/shortest constructor name starting with `:`. – chepner Jul 02 '19 at 18:42

What is the reason of error "Not a data constructor"?

1 Answers1