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Internal error when rewriting type constructors #3846

jespercockx opened this issue Jun 5, 2019 · 0 comments


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@jespercockx jespercockx commented Jun 5, 2019

This has been known for a while, but lately I have been reconsidering the kind of guarantees one should be able to expect from rewrite rules, and it bothers me that it's so easy to trigger internal errors:

{-# OPTIONS --rewriting #-}

open import Agda.Builtin.Equality
open import Agda.Builtin.Bool
open import Agda.Builtin.Nat


not : Bool  Bool
not true = false
not false = true

postulate rew : Nat ≡ Bool
{-# REWRITE rew #-}

0' : Bool
0' = 0

test : not 0' ≡ true
test = refl

Error message:

An internal error has occurred. Please report this as a bug.
Location of the error: src/full/Agda/TypeChecking/Reduce/Fast.hs:1347

I'm thinking that we should maybe not allow rewrite rules on type constructors by default.

@jespercockx jespercockx added this to the 2.6.1 milestone Jun 5, 2019
@jespercockx jespercockx self-assigned this Jun 5, 2019
jespercockx added a commit that referenced this issue Jun 10, 2019
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