Reader monad transform one env to another

Description

Hi, I’ d like to chain two reader monads, where one env is subtype of the other, so inner function can access only type it requires and outer function can requires general env param.

Repro steps

type Env = {Now: DateTime; Conn: string}

type InnerEnv = {Now: DateTime}

let inner () =
    monad {
        let! (e:InnerEnv) = Reader.ask
        return doWork e.Now
    }

let outer () =
    monad {
        let! (e:Env) = Reader.ask
        let! m = Reader (fun p -> {Now = p.Now})
        let! r = inner()
        let! m = Reader.local (fun (x:Env) -> {Now = x.Now})
        return r
    }

Expected behavior

Expected result is function outer with signature Reader<Env, Result>

Actual behavior

Doesn’t compile

[padzikm]

I found solution:

type Env = {Now: DateTime; Conn: string}

type InnerEnv = {Now: DateTime}

let inner () =
    monad {
        let! (e:InnerEnv) = Reader.ask
        return doWork e.Now
    }

let outer () =
    monad {
        let! r1 = inner() |> Reader.local (fun (x:Env) -> {Now = x.Now})
        let! r2 = otherInner() |> Reader.local (fun (x:Env) -> {Conn = x.Conn})
        return r1, r2
    }

[gusty]

I see, it looks like I misunderstood your problem.

I was thinking in having a generic env with some constraints, and I came up once with a solution which requires SRTP accessor functions for each property, which is something like:

let inline getConn x = (^T : (member Conn : _) x)

but as I said, this is not what you need.