MR K P SCHUPKE writes:

Try type annotations:

[slightly edited to keep the different s's differentiated]

new_point (s0::s) t b : interleave (next_state s0::s) bs (t:ts)

Hmm...I get: Inferred type is less polymorphic than expected Quantified type variable `s' escapes It is mentioned in the environment: s0 :: s (bound at Grid.lhs:25) When checking the type signature of the expression: s0 :: forall s. s -kzm -- If I haven't seen further, it is by standing in the footprints of giants