Support constructors in rewrite rules

[celso]

When trying to bind to an existing datatype (like Aeson.Value, for example), I believe I should add a custom rewrite rule, like:

rewrites:
  - from: "Data.AesonBind.Value"
    to: "Value"
    importing: "Data.Aeson"
  - from: "Data.AesonBind.Number"
    to: "Number"
    importing: "Data.Aeson"

When compiled, using Number will result in the following import:

import Data.Aeson (Number, Value)

But this results in a compile error, as the code should be

import Data.Aeson (Value(Number))

I'm not sure if support this kind of use-case makes sense for the project, but IMO it's a good feature to have when supporting binding to existing Haskell.

A possible solution I was thinking about would be adding an optional constructorOf field to the rewrite rules, so we could write it in the following way:

rewrites:
  - from: "Data.AesonBind.Value"
    to: "Value"
    importing: "Data.Aeson"
  - from: "Data.AesonBind.Number"
    to: "Number"
    importing: "Data.Aeson"
    constructorOf: "Value"

And it would generate the proper import.

I'd be happy to work on this if the feature makes sense 😊

[Lucas Escot]

I think agda2hs should definitely allow rewrite rules to constructors!

Though personally I would refrain from adding more fields to rewrite rules in the config file.

Perhaps we could make to : "Number.Value" mean "constructor Value of datatype Number" or something.

Waiting for @jespercockx to weigh in.

I don't know if we support rewrite rules over infix operators. In general we don't have any tests currently for rewrite rules, which should be fixed.

[Jesper Cockx]

I like the suggestion by @flupe , and I agree that we need test cases for rewrite rules.