Termination checking prefers `data` to `record`
lawcho opened this issue · comments
foldSS passes when data is used, but not when record is used:
-- Fails syntax-based termination checking
record SillyStream (A : Set) : Set where
inductive; eta-equality; constructor _∷_
field head : A
field tail : SillyStream A
-- -- Passes syntax-based termination checking
-- data SillyStream (A : Set) : Set where
-- _∷_
-- : (head : A)
-- → SillyStream A
-- → SillyStream A
foldSS : {A r : Set} → (A → r → r) → SillyStream A → r
foldSS f (head ∷ tail) = f head (foldSS f tail)I expected syntax-based termination to spot the structural recursion either way.
Agda Versions tested
It should work if you put no-eta-equality.
With eta, foldSS is not terminating due to the record pattern translation which turns this into
foldSS f s = f (head s) (foldSS f (tail s))This passes:
record SillyStream (A : Set) : Set where
inductive; pattern; constructor _∷_
field head : A
field tail : SillyStream A
foldSS : {A r : Set} → (A → r → r) → SillyStream A → r
foldSS f (head ∷ tail) = f head (foldSS f tail)Yes, because eta is off. This gives an error:
...
module _ (A : Set) (s : SillyStream A) where
eta : s ≡ head s ∷ tail s
eta = reflIt seems like pattern is enough to get data-style checking of a record.
I don't think a doc update is required, since the pattern keyword is discoverable via Agda's error message:
/tmp/tmp.O6kGsXwZPr/tmp.agda:8,11-22
Pattern matching on no-eta record type SillyStream
(defined at /tmp/tmp.O6kGsXwZPr/tmp.agda:2,8-19) is not allowed
(to activate, add declaration `pattern` to record definition)
when checking that the pattern head ∷ tail has type SillyStream A
which arises when I give a naive translation of the data type:
record SillyStream (A : Set) : Set where
inductive; constructor _∷_
field head : A
field tail : SillyStream A(Agda version 422b932 of master)