unify (ListType t1) (ListType t2) = unify t1 t2 >>| (pure $ ListType t1)
unify (ta1 ->> ta2) (tb1 ->> tb2) = unify ta1 tb1 >>= \ta-> unify ta2 tb2
>>= \tb-> pure (ta ->> tb)
unify (ListType t1) (ListType t2) = unify t1 t2 >>| (pure $ ListType t1)
unify (ta1 ->> ta2) (tb1 ->> tb2) = unify ta1 tb1 >>= \ta-> unify ta2 tb2
>>= \tb-> pure (ta ->> tb)