1311e36a98
This adds support for mutual structural recursive functions. For now this is opt-in: The functions must have a `termination_by structural …` annotation (new since #4542) for this to work: ```lean mutual inductive A | self : A → A | other : B → A | empty inductive B | self : B → B | other : A → B | empty end mutual def A.size : A → Nat | .self a => a.size + 1 | .other b => b.size + 1 | .empty => 0 termination_by structural x => x def B.size : B → Nat | .self b => b.size + 1 | .other a => a.size + 1 | .empty => 0 termination_by structural x => x end ``` The recursive functions don’t have to be in a one-to-one relation to a set of mutually recursive inductive data types. It is possible to ignore some of the types: ```lean def A.self_size : A → Nat | .self a => a.self_size + 1 | .other _ => 0 | .empty => 0 termination_by structural x => x ``` or have more than one function per argument type: ```lean def isEven : Nat → Prop | 0 => True | n+1 => ¬ isOdd n termination_by structural x => x def isOdd : Nat → Prop | 0 => False | n+1 => ¬ isEven n termination_by structural x => x ``` This does not include * Support for nested inductive data types or nested recursion * Inferring mutual structural recursion in the absence of `termination_by`. * Functional induction principles for these. * Mutually recursive functions that live in different universes. This may be possible, maybe after beefing up the `.below` and `.brecOn` functions; we can look into this some other time, maybe when there are concrete use cases. --------- Co-authored-by: Richard Kiss <him@richardkiss.com> Co-authored-by: Tobias Grosser <tobias@grosser.es> |
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