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fix' f = d d
  where
    d x = f (\t -> x x t)
 
main = print 1

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:3:11: error:
    • Couldn't match expected type ‘(t2 -> t3) -> t4’
                  with actual type ‘p’
        because type variables ‘t2’, ‘t3’, ‘t4’ would escape their scope
      These (rigid, skolem) type variables are bound by
        the inferred type of d :: (t1 -> t2 -> t3) -> t4
        at prog.hs:3:5-25
    • In the expression: f (\ t -> x x t)
      In an equation for ‘d’: d x = f (\ t -> x x t)
      In an equation for ‘fix'’:
          fix' f
            = d d
            where
                d x = f (\ t -> x x t)
    • Relevant bindings include
        x :: t1 -> t2 -> t3 (bound at prog.hs:3:7)
        d :: (t1 -> t2 -> t3) -> t4 (bound at prog.hs:3:5)
        f :: p (bound at prog.hs:1:6)
        fix' :: p -> t (bound at prog.hs:1:1)
  |
3 |     d x = f (\t -> x x t)
  |           ^^^^^^^^^^^^^^^

prog.hs:3:22: error:
    • Occurs check: cannot construct the infinite type:
        t1 ~ t1 -> t2 -> t3
    • In the first argument of ‘x’, namely ‘x’
      In the expression: x x t
      In the first argument of ‘f’, namely ‘(\ t -> x x t)’
    • Relevant bindings include
        t :: t2 (bound at prog.hs:3:15)
        x :: t1 -> t2 -> t3 (bound at prog.hs:3:7)
        d :: (t1 -> t2 -> t3) -> t4 (bound at prog.hs:3:5)
  |
3 |     d x = f (\t -> x x t)
  |                      ^

Standard output is empty