Generic put doesn't work in Monad Transformers about fsharpplus HOT 3 CLOSED

Done here: dotnet/fsharp#7384

from fsharpplus.

This is a consequence of another issue: type inference fails when liftM is implemented as map.
At the same time that issue is a consequence of an F# issue, since after doing some tests like changing the definition like this:

static member inline Lift (x:'``Monad<'T>``) = 
  if false then ListT (x >>= (result << List.singleton)) else ListT (map List.singleton x)

and then trying to call with the repro code it looks like type inference fails depending on the execution path (switch to true and it compiles fine).

This is clearly an F# bug, compilation shouldn't fail depending on the execution path.

TODO:

• Find a minimal repro
• Report it as a bug in the F# compiler project, then reference this issue and close it
• If no one there is interested in solving it, try to fix it

In the meantime the best compromise is to re-re-introduce liftM this might lead to a slightly less optimised code for those cases where the map operation is more efficient but at least will work which is a minimal requirement.

If more efficient code is required it's always possible to replace the generic code with a more specific implementation, in the repro it will be:

let x = ListT (map List.singleton (State.put initialState))

from fsharpplus.

Reopening as this is still an issue

from fsharpplus.