[Agda] Strangeness with corecursion

[Thorsten Altenkirch]

Hi,

I just set the exercise to define multiplication for conats for my course on type theory. So please don’t blurt put the solution – see the attached code. The following is not a solution because agda doesn’t see that the definition is productive (even though it is):

prd∞ (m *∞ n) with prd∞ m
prd∞ (m *∞ n) | (just pm) = prd∞ (n +∞ (pm *∞ n))
prd∞ (m *∞ n) | nothing   = nothing

but after the following change

prd∞ (m *∞ n) with prd∞ m
prd∞ (m *∞ n) | (just pm) with (pm *∞ n)
prd∞ (m *∞ n) | (just pm) | x = prd∞ (n +∞ x)
prd∞ (m *∞ n) | nothing   = nothing

agda seems to accept is. Actually even if +∞ is not contractive.

Is this a known bug? Otherwise can somebody remind e how to report this?

Cheers,
Thorsten



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