[Agda] strange "Unreachable clause"

[Ulf Norell]

0# and 1# are not constructors that you can match against. You can tell
from them being highlighted in a different colour than d1 and d2.

It wouldn't make sense to be able to match on 0# and 1# since depending
on the semiring they could be the same value, or have a type that doesn't
have decidable equality.

/ Ulf

On Wed, May 2, 2018 at 12:35 PM, Sergei Meshveliani <mechvel at botik.ru>
wrote:

> Please, what is wrong in f' below?
> Why Agda-2.5.3 type-checks f and does not type-check f' ?
>
> ---------------------------------------------------------
> open import Level    using () renaming (zero to 0ℓ)
> open import Algebra  using (Semiring)
> open import Data.Nat using (ℕ)
>
> data Expr : Set  where c1 : ℕ → Expr
>                        c2 : Expr → Expr → Expr
> f : Expr → ℕ
> f (c1 _)        =  0
> f (c2 (c1 0) _) =  0
> f (c2 (c1 1) _) =  0
> f (c2 _      _) =  0
>
> postulate R : Semiring 0ℓ 0ℓ
> open Semiring R using (Carrier; 0#; 1#)
>
> data Expr' : Set  where d1 : Carrier → Expr'
>                         d2 : Expr' → Expr' → Expr'
> f' : Expr' → ℕ
> f' (d1 _)         =  0
> f' (d2 (d1 0#) _) =  0
> f' (d2 (d1 1#) _) =  0
> f' (d2 _       _) =  0
> ----------------------------------------------------------
>
> Agda reports  Unreachable clause  for the third pattern in  f',  as if
> it was shadowed by previous pattern. But it is not shadowed.
>
> Is this a bug in Agda-2.5.3 ?
>
> ------
> Sergei
>
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