3ab2c714ec
the support for mutual structural recursion (new since #4575) is extended so that Lean tries to infer it even without annotations. * The error message when termination checking fails looks quite different now. Maybe a bit better, maybe with more room for improvements. * If there are too many combinations (with an arbitrary cut-off) for a given argument type, it will just give up and ask the user to use `termination_by structural`. * It is now legal to specify `termination_by structural` on not necessarily all functions of a clique; this simply restricts the combinations of arguments that Lean considers. --------- Co-authored-by: Tobias Grosser <tobias@grosser.es>
49 lines
1.5 KiB
Text
49 lines
1.5 KiB
Text
set_option linter.constructorNameAsVariable false
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inductive A (n : Nat) : Type
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| a : A n
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| b : A n → A n
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/--
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error: cannot use specified parameter for structural recursion:
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its type is an inductive datatype
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A n
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and the datatype parameter
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n
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depends on the function parameter
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n
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which does not come before the varying parameters and before the indices of the recursion parameter.
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-/
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#guard_msgs in
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def A.size (acc n : Nat) : A n → Nat
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| .a => acc
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| .b a' => 1 + A.size (acc + 1) n a'
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termination_by structural a => a
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-- Another instance repoted on Zulip at
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-- https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/.E2.9C.94.20Doubly-nested.20inductive/near/451204850
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inductive Xn (e : Sigma.{0} (· → Type)) (α : Type) : Nat → Type where
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| mk1_S {n} (x : Xn e α n) : Xn e α (n+1)
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| mk2 {n} (s : e.1) (p : e.2 s → Xn e α n) : Xn e α n
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/--
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error: cannot use specified parameter for structural recursion:
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its type is an inductive datatype
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Xn e (Xn e α n) m
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and the datatype parameter
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Xn e α n
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depends on the function parameter
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n
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which does not come before the varying parameters and before the indices of the recursion parameter.
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-/
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#guard_msgs in
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def Xn.zip {e α m n} : Xn e (Xn e α n) m → Xn e α (n+m+1)
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| .mk1_S x => .mk1_S x.zip
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| .mk2 s p => .mk2 s fun a => (p a).zip
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termination_by structural x => x
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def Xn.zip' {e α n m} : Xn e (Xn e α n) m → Xn e α (n+m+1)
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| .mk1_S x => .mk1_S x.zip'
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| .mk2 s p => .mk2 s fun a => (p a).zip'
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termination_by structural x => x
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