acfc11ae42
This PR fixes an issue where nested `let rec` declarations within `match` expressions or tactic blocks failed to compile if they were nested within, and recursively called, a `let rec` that referenced a variable bound by a containing declaration. Closes #6927 --------- Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
93 lines
2.7 KiB
Text
93 lines
2.7 KiB
Text
import Lean
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/-
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# Nested `let rec` declarations in the presence of delayed metavariable assignments
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https://github.com/leanprover/lean4/issues/6927
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Nested declarations using `let rec` should compile correctly even when nested within expressions
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that are elaborated using delayed metavariable assignments, such as `match` expressions and tactic
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blocks. See the Note [Delayed-Assigned Metavariables in Free Variable Collection] in
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`MutualDef.lean` for additional details.
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-/
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inductive Tree : Type where
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| leaf
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| node (p : Nat) (brs : List Tree)
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def Tree.substNats (M : Tree) (σ : Nat → Nat) : Tree :=
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let rec go M : Tree := match M with
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| .leaf => leaf
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| .node p brs =>
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let rec goList : List Tree → List Tree
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| [] => []
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| M :: Ms => go M :: goList Ms
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.node (σ p) (goList brs)
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go M
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/-
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Previously, free variable dependencies in `inner` were not correctly detected, leading that
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declaration to contain free variables when sent to the kernel.
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-/
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namespace Ex1
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def outer (f : Nat → Nat) :=
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let rec middle n := match n with
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| Nat.zero => 0 | .succ _ =>
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let rec inner := middle n
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f 0
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()
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end Ex1
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/-
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`by` blocks induced behavior similar to the above, since they also produce metavariables during
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elaboration.
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-/
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def tac (f : Nat → Nat) :=
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let rec middle n := by
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let rec inner := middle (Nat.succ n)
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exact f 0
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()
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/- Previously, `inner` was incorrectly compiled as an `axiom`, rendering `outer` noncomputable. -/
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namespace Ex2
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/--
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error: fail to show termination for
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Ex2.outer.middle.inner
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Ex2.outer.middle
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with errors
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failed to infer structural recursion:
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Not considering parameter f of Ex2.outer.middle.inner:
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it is unchanged in the recursive calls
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Not considering parameter f of Ex2.outer.middle:
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it is unchanged in the recursive calls
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Cannot use parameters n✝ of Ex2.outer.middle.inner and n of Ex2.outer.middle:
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failed to eliminate recursive application
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outer.middle f✝ (n + 1)
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Cannot use parameters n of Ex2.outer.middle.inner and n of Ex2.outer.middle:
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failed to eliminate recursive application
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outer.middle f✝ (n + 1)
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Could not find a decreasing measure.
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The basic measures relate at each recursive call as follows:
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(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
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Call from Ex2.outer.middle.inner to Ex2.outer.middle at 89:6-20:
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n✝ n
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n ? ?
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Call from Ex2.outer.middle to Ex2.outer.middle.inner at 91:4-11:
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n
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n✝ _
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n _
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Please use `termination_by` to specify a decreasing measure.
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-/
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#guard_msgs in
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def outer (f : Nat → Nat) (n : Nat) : Nat :=
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let rec middle n : Nat := match n with
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| 0 => 1
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| n + 1 =>
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let rec inner (n : Nat) : Nat :=
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middle (n + 1)
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let _ := f
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inner n
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middle n
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end Ex2
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