Summary

Let statements don't work with ApplicativeDo. Bug or feature?

Steps to reproduce

Compile this snippet and see the compiler complain.

{-# LANGUAGE ApplicativeDo #-}
module Bug where

foo :: Applicative f => f Int
foo = undefined

bar :: Applicative f => f Int
bar = do
  x <- foo
  let y = x + 1
  pure y

Expected behavior

I would expect let bound variables to be allowed if they are not used in any RHSs of <- bindings.

The example above can trivially bee rewritten to (which works)

bar :: Applicative f => f Int
bar = do
  x <- foo
  pure $ let y = x + 1 in y

Environment

• GHC version used:

The Glorious Glasgow Haskell Compilation System, version 8.10.1