{-# LANGUAGE MonadComprehensions, ParallelListComp #-}

module Foo where

foo xs ys = [ (f y True, f x 'c')
            | let f _ z = z, x <- xs
            | y <- ys ]

This fails with

Foo.hs:5:52: error:
    * Cannot instantiate unification variable `t0'
      with a type involving foralls: forall t2 t3. t3 -> t2 -> t2
        GHC doesn't yet support impredicative polymorphism
    * In a stmt of a monad comprehension:
        let f _ z = z, x <- xs |  y <- ys
      In the expression:
        [(f y True, f x 'c') | let f _ z = z, x <- xs |  y <- ys]
      In an equation for `foo':
          foo xs ys
            = [(f y True, f x 'c') | let f _ z = z, x <- xs |  y <- ys]

NB: ApplicativeDo has a related problem: the implementation is quite different and has the effect of monomorphising the let-bound variable.