# [Agda] coverage in Agda-2.1.3

Thorsten Altenkirch txa at Cs.Nott.AC.UK
Wed May 7 11:54:36 CEST 2008

```Hi,

I have just installed 2.1.3 on my desktop, while my laptop still runs
2.1.2. It seems that the coverage checker has become more fascist. The
following definition was accepted on 2.1.2

fpair : forall {m n} -> Fin m -> Fin n -> Fin (m × n)
fpair {succ m} {n} fz     i = finl {n} {m × n} i
fpair {succ m} {n} (fs j) i = finr {n} {m × n} (fpair j i)

but is rejected on 2.1.3:

Incomplete pattern matching for fpair. Missing cases:
fpair {zero} {_} _ _
when checking the definition of fpair

I don't agree since fz, fs already constitute a cover. In any case,
how would I fill in this case? I tried

fpair : forall {m n} -> Fin m -> Fin n -> Fin (m × n)
fpair {zero}   {n} i      ()
fpair {succ m} {n} fz     i = finl {n} {m × n} i
fpair {succ m} {n} (fs j) i = finr {n} {m × n} (fpair j i)

but Agda says (in the ghci buffer !?)

Fin n should be empty, but it isn't (as far as I can see)
when checking that the clause fpair {zero} {n} i () has type ...

It seems that I have the choice to revert to 2.1.2 or:

impossible : {A : Set} -> Fin zero -> A
impossible ()

fpair : forall {m n} -> Fin m -> Fin n -> Fin (m × n)
fpair {zero}   {n} j      _ = impossible j
fpair {succ m} {n} fz     i = finl {n} {m × n} i
fpair {succ m} {n} (fs j) i = finr {n} {m × n} (fpair j i)

Cheers,
Thorsten
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