[Haskell-cafe] hs-boot and (micro)lenses

2 Aug 2018

      Hello cafe,
    I have some problem setting up an appropriate .hs-boot module.
I attach a .tar.gz'd cabal folder, so you can immediately test it
with `cabal new-repl`.

I had a file like this

    -- datatypes
    data A = A { _i :: Int,
             _b :: B }
    type AS a = State A a

    data B = B (AS ())

    makeLenses ''A

    -- functions
    f :: B
    f = B $ i .= 18

So I decided to split it up in 2 files to test hs-boot in the wild:

    -- A.hs
    -------
    data A = A { _i :: Int,
             _b :: B }
    type AS a = State A a

    makeLenses ''A

    -- B.hs
    -------
    data B = B (AS ())

    f :: B
    f = B $ i .= 18

Well, I don't know how to write A.hs-boot. I tried this:

    module A where

    import Lens.Micro.Platform

    data A

    b :: Lens' A B

but ghc rightfully complains that he doesn't know about B, and if
I slap an `import B` in the .hs-boot then I have another import-cycle.

What's the way out of that?
The project I am dealing with is bigger than this problem and I would
really love to keep functions and related datatypes nearby for mental
sanity.

As I wrote above, attached you can find a .tar.gz with everything set
up to replicate this
-F

[Haskell-cafe] hs-boot and (micro)lenses

Francesco Ariis