v4.9.0-rc1

chore: set LEAN_VERSION_IS_RELEASE to 1

v4.8.0

Language features, tactics, and metaprograms

• Functional induction principles.
  #3432, #3620, #3754, #3762, #3738, #3776, #3898.

  Derived from the definition of a (possibly mutually) recursive function,
  a functional induction principle is created that is tailored to proofs about that function.

  For example from:

  def ackermann : Nat → Nat → Nat
    | 0, m => m + 1
    | n+1, 0 => ackermann n 1
    | n+1, m+1 => ackermann n (ackermann (n + 1) m)
  

  we get

  ackermann.induct (motive : Nat → Nat → Prop) (case1 : ∀ (m : Nat), motive 0 m)
    (case2 : ∀ (n : Nat), motive n 1 → motive (Nat.succ n) 0)
    (case3 : ∀ (n m : Nat), motive (n + 1) m → motive n (ackermann (n + 1) m) → motive (Nat.succ n) (Nat.succ m))
    (x x : Nat) : motive x x
  

  It can be used in the induction tactic using the using syntax:

  induction n, m using ackermann.induct
  

• The termination checker now recognizes more recursion patterns without an
  explicit termination_by. In particular the idiom of counting up to an upper
  bound, as in

  def Array.sum (arr : Array Nat) (i acc : Nat) : Nat :=
    if _ : i < arr.size then
      Array.sum arr (i+1) (acc + arr[i])
    else
      acc
  

  is recognized without having to say termination_by arr.size - i.

  • #3630 makes termination_by? not use sizeOf when not needed
  • #3652 improves the termination_by syntax.
  • #3658 changes how termination arguments are elaborated.
  • #3665 refactors GuessLex to allow inferring more complex termination arguments
  • #3666 infers termination arguments such as xs.size - i

• #3629, #3655, #3747:
  Adds @[induction_eliminator] and @[cases_eliminator] attributes to be able to define custom eliminators
  for the induction and cases tactics, replacing the @[eliminator] attribute.
  Gives custom eliminators for Nat so that induction and cases put goal states into terms of 0 and n + 1
  rather than Nat.zero and Nat.succ n.
  Added option tactic.customEliminators to control whether to use custom eliminators.
  Added a hack for rcases/rintro/obtain to use the custom eliminator for Nat.

• Shorter instances names. There is a new algorithm for generating names for anonymous instances.
  Across Std and Mathlib, the median ratio between lengths of new names and of old names is about 72%.
  With the old algorithm, the longest name was 1660 characters, and now the longest name is 202 characters.
  The new algorithm's 95th percentile name length is 67 characters, versus 278 for the old algorithm.
  While the new algorithm produces names that are 1.2% less unique,
  it avoids cross-project collisions by adding a module-based suffix
  when it does not refer to declarations from the same "project" (modules that share the same root). #3089 and #3934.

• 8d2adf Importing two different files containing proofs of the same theorem is no longer considered an error.
  This feature is particularly useful for theorems that are automatically generated on demand (e.g., equational theorems).

• 84b091 Lean now generates an error if the type of a theorem is not a proposition.

• Definition transparency. 47a343. @[reducible], @[semireducible], and @[irreducible] are now scoped and able to be set for imported declarations.

• simp/dsimp

  • #3607 enables kernel projection reduction in dsimp
  • b24fbf and acdb00: dsimproc command to define defeq-preserving simplification procedures.
  • #3624 makes dsimp normalize raw nat literals as OfNat.ofNat applications.
  • #3628 makes simp correctly handle OfScientific.ofScientific literals.
  • #3654 makes dsimp? report used simprocs.
  • dee074 fixes equation theorem handling in simp for non-recursive definitions.
  • #3819 improved performance when simp encounters a loop.
  • #3821 fixes discharger/cache interaction.
  • #3824 keeps simp from breaking Char literals.
  • #3838 allows Nat instances matching to be more lenient.
  • #3870 documentation for simp configuration options.
  • #3972 fixes simp caching.
  • #4044 improves cache behavior for "well-behaved" dischargers.

• omega

  • #3639, #3766, #3853, #3875: introduces a term canonicalizer.
  • #3736 improves handling of positivity for the modulo operator for Int.
  • #3828 makes it work as a simp discharger.
  • #3847 adds helpful error messages.

• rfl

  • #3671, #3708: upstreams the @[refl] attribute and the rfl tactic.
  • #3751 makes apply_rfl not operate on Eq itself.
  • #4067 improves error message when there are no goals.

• #3719 upstreams the rw? tactic, with fixes and improvements in #3783, #3794, #3911.

• conv

  • #3659 adds a conv version of the calc tactic.
  • #3763 makes conv clean up using try with_reducible rfl instead of try rfl.

• #guard_msgs

  • #3617 introduces whitespace protection using the ⏎ character.
  • #3883: The #guard_msgs command now has options to change whitespace normalization and sensitivity to message ordering.
    For example, #guard_msgs (whitespace := lax) in cmd collapses whitespace before checking messages,
    and #guard_msgs (ordering := sorted) in cmd sorts the messages in lexicographic order before checking.
  • #3931 adds an unused variables ignore function for #guard_msgs.
  • #3912 adds a diff between the expected and actual outputs. This feature is currently
    disabled by default, but can be enabled with set_option guard_msgs.diff true.
    Depending on user feedback, this option may default to true in a future version of Lean.

• do notation

  • #3820 makes it an error to lift (<- ...) out of a pure if ... then ... else ...

• Lazy discrimination trees

  • #3610 fixes a name collision for LazyDiscrTree that could lead to cache poisoning.
  • #3677 simplifies and fixes LazyDiscrTree handling for exact?/apply?.
  • #3685 moves general exact?/apply? functionality into LazyDiscrTree.
  • #3769 has lemma selection improvements for rw? and LazyDiscrTree.
  • #3818 improves ordering of matches.

• #3590 adds inductive.autoPromoteIndices option to be able to disable auto promotion of indices in the inductive command.

• Miscellaneous bug fixes and improvements

  • #3606 preserves cache and dischargeDepth fields in Lean.Meta.Simp.Result.mkEqSymm.
  • #3633 makes elabTermEnsuringType respect errToSorry, improving error recovery of the have tactic.
  • [#3647](https://github.com/leanprover/lean4/pull/...

v4.8.0-rc2

Update to v4.8.0-rc1 with improvements to lake's build monitor.

v4.8.0-rc1

chore: set release flag

v4.7.0

Changes since v4.6.0 (from RELEASES.md)

• simp and rw now use instance arguments found by unification,
  rather than always resynthesizing. For backwards compatibility, the original behaviour is
  available via set_option tactic.skipAssignedInstances false.
  #3507 and
  #3509.

• When the pp.proofs is false, now omitted proofs use ⋯ rather than _,
  which gives a more helpful error message when copied from the Infoview.
  The pp.proofs.threshold option lets small proofs always be pretty printed.
  #3241.

• pp.proofs.withType is now set to false by default to reduce noise in the info view.

• The pretty printer for applications now handles the case of over-application itself when applying app unexpanders.
  In particular, the | `($_ $a $b $xs*) => `(($a + $b) $xs*) case of an app_unexpander is no longer necessary.
  #3495.

• New simp (and dsimp) configuration option: zetaDelta. It is false by default.
  The zeta option is still true by default, but their meaning has changed.

  • When zeta := true, simp and dsimp reduce terms of the form
    let x := val; e[x] into e[val].
  • When zetaDelta := true, simp and dsimp will expand let-variables in
    the context. For example, suppose the context contains x := val. Then,
    any occurrence of x is replaced with val.

  See issue #2682 for additional details. Here are some examples:

  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    simp
    /-
    New goal:
    h : z = 9; x := 5 |- x + 4 = z
    -/
    rw [h]
  
  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    -- Using both `zeta` and `zetaDelta`.
    simp (config := { zetaDelta := true })
    /-
    New goal:
    h : z = 9; x := 5 |- 9 = z
    -/
    rw [h]
  
  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    simp [x] -- asks `simp` to unfold `x`
    /-
    New goal:
    h : z = 9; x := 5 |- 9 = z
    -/
    rw [h]
  
  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    simp (config := { zetaDelta := true, zeta := false })
    /-
    New goal:
    h : z = 9; x := 5 |- let y := 4; 5 + y = z
    -/
    rw [h]
  

• When adding new local theorems to simp, the system assumes that the function application arguments
  have been annotated with no_index. This modification, which addresses issue #2670,
  restores the Lean 3 behavior that users expect. With this modification, the following examples are now operational:

  example {α β : Type} {f : α × β → β → β} (h : ∀ p : α × β, f p p.2 = p.2)
    (a : α) (b : β) : f (a, b) b = b := by
    simp [h]
  
  example {α β : Type} {f : α × β → β → β}
    (a : α) (b : β) (h : f (a,b) (a,b).2 = (a,b).2) : f (a, b) b = b := by
    simp [h]
  

  In both cases, h is applicable because simp does not index f-arguments anymore when adding h to the simp-set.
  It's important to note, however, that global theorems continue to be indexed in the usual manner.

• Improved the error messages produced by the decide tactic. #3422

• Improved auto-completion performance. #3460

• Improved initial language server startup performance. #3552

• Changed call hierarchy to sort entries and strip private header from names displayed in the call hierarchy. #3482

• There is now a low-level error recovery combinator in the parsing framework, primarily intended for DSLs. #3413

• You can now write termination_by? after a declaration to see the automatically inferred
  termination argument, and turn it into a termination_by … clause using the “Try this” widget or a code action. #3514

• A large fraction of Std has been moved into the Lean repository.
  This was motivated by:

  1. Making universally useful tactics such as ext, by_cases, change at,
     norm_cast, rcases, simpa, simp?, omega, and exact?
     available to all users of Lean, without imports.
  2. Minimizing the syntactic changes between plain Lean and Lean with import Std.
  3. Simplifying the development process for the basic data types
     Nat, Int, Fin (and variants such as UInt64), List, Array,
     and BitVec as we begin making the APIs and simp normal forms for these types
     more complete and consistent.
  4. Laying the groundwork for the Std roadmap, as a library focused on
     essential datatypes not provided by the core langauge (e.g. RBMap)
     and utilities such as basic IO.
     While we have achieved most of our initial aims in v4.7.0-rc1,
     some upstreaming will continue over the coming months.

• The / and % notations in Int now use Int.ediv and Int.emod
  (i.e. the rounding conventions have changed).
  Previously Std overrode these notations, so this is no change for users of Std.
  There is now kernel support for these functions.
  #3376.

• omega, our integer linear arithmetic tactic, is now availabe in the core langauge.

  • It is supplemented by a preprocessing tactic bv_omega which can solve goals about BitVec
    which naturally translate into linear arithmetic problems.
    #3435.
  • omega now has support for Fin #3427,
    the <<< operator #3433.
  • During the port omega was modified to no longer identify atoms up to definitional equality
    (so in particular it can no longer prove id x ≤ x). #3525.
    This may cause some regressions.
    We plan to provide a general purpose preprocessing tactic later, or an omega! mode.
  • omega is now invoked in Lean's automation for termination proofs
    #3503 as well as in
    array indexing proofs #3515.
    This automation will be substantially revised in the medium term,
    and while omega does help automate some proofs, we plan to make this much more robust.

• The library search tactics exact? and apply? that were originally in
  Mathlib are now available in Lean itself. These use the implementation using
  lazy discrimination trees from Std, and thus do not require a disk cache but
  have a slightly longer startup time. The order used for selection lemmas has
  changed as well to favor goals purely based on how many terms in the head
  pattern match the current goal.

• The solve_by_elim tactic has been ported from Std to Lean so that library
  search can use it.

• New #check_tactic and #check_simp commands have been added. These are
  useful for checking tactics (particularly simp) behave as expected in test
  suites.

• Previously, app unexpanders would only be applied to entire applications. However, some notations produce
  functions, and these functions can be given additional arguments. The solution so far has been to write app unexpanders so that they can take an arbitrary number of additional arguments. However this leads to misleading hover information in the Infoview. For example, while HAdd.hAdd f g 1 pretty prints as (f + g) 1, hovering over f + g shows f. There is no way to fix the situation from within an app unexpander; the expression position for HAdd.hAdd f g is absent, and app unexpanders cannot register TermInfo.

  This commit changes the app delaborator to try running app unexpanders on every prefix of an application, from longest to shortest prefix. For efficiency, it is careful to only try this when app delaborators do in fact exist for the head constant, and it also ensures arguments are only delaborated once. Then, in (f + g) 1, the f + g gets TermInfo registered for that subexpression, making it properly hoverable.

  #3375

Breaking changes:

• Lean.withTraceNode and variants got a stronger MonadAlwaysExcept assumption to
  fix trace trees not being built on elaboration runtime exceptions. Instances for most elaboration
  monads built on EIO Exception should be synthesized automatically.
• The match ... with. and fun. notations previously in Std have been replaced by
  nomatch ... and nofun. #3279 and #3286

Other improvements:

• several bug fixes for simp:
  • we should not crash when simp loops #3269
  • simp gets stuck on autoParam #3315
  • simp fails when custom discharger makes no progress #3317
  • simp fails to discharge autoParam premises even when it can reduce them to True #3314
  • simp? suggests generated equations lemma names, fixes #3547 #3573
• fixes for match expressions:
  • fix regression with builtin literals #3521...

v4.7.0-rc2

Identical to v4.7.0-rc1, but with the backport of #3610, fixing a bug when exact? is used in Mathlib.

v4.7.0-rc1

Changes since v4.6.0 (from RELEASES.md)

• simp and rw now use instance arguments found by unification,
  rather than always resynthesizing. For backwards compatibility, the original behaviour is
  available via set_option tactic.skipAssignedInstances false.
  #3507 and
  #3509.

• When the pp.proofs is false, now omitted proofs use ⋯ rather than _,
  which gives a more helpful error message when copied from the Infoview.
  The pp.proofs.threshold option lets small proofs always be pretty printed.
  #3241.

• pp.proofs.withType is now set to false by default to reduce noise in the info view.

• The pretty printer for applications now handles the case of over-application itself when applying app unexpanders.
  In particular, the | `($_ $a $b $xs*) => `(($a + $b) $xs*) case of an app_unexpander is no longer necessary.
  #3495.

• New simp (and dsimp) configuration option: zetaDelta. It is false by default.
  The zeta option is still true by default, but their meaning has changed.

  • When zeta := true, simp and dsimp reduce terms of the form
    let x := val; e[x] into e[val].
  • When zetaDelta := true, simp and dsimp will expand let-variables in
    the context. For example, suppose the context contains x := val. Then,
    any occurrence of x is replaced with val.

  See issue #2682 for additional details. Here are some examples:

  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    simp
    /-
    New goal:
    h : z = 9; x := 5 |- x + 4 = z
    -/
    rw [h]
  
  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    -- Using both `zeta` and `zetaDelta`.
    simp (config := { zetaDelta := true })
    /-
    New goal:
    h : z = 9; x := 5 |- 9 = z
    -/
    rw [h]
  
  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    simp [x] -- asks `simp` to unfold `x`
    /-
    New goal:
    h : z = 9; x := 5 |- 9 = z
    -/
    rw [h]
  
  example (h : z = 9) : let x := 5; let y := 4; x + y = z := by
    intro x
    simp (config := { zetaDelta := true, zeta := false })
    /-
    New goal:
    h : z = 9; x := 5 |- let y := 4; 5 + y = z
    -/
    rw [h]
  

• When adding new local theorems to simp, the system assumes that the function application arguments
  have been annotated with no_index. This modification, which addresses issue #2670,
  restores the Lean 3 behavior that users expect. With this modification, the following examples are now operational:

  example {α β : Type} {f : α × β → β → β} (h : ∀ p : α × β, f p p.2 = p.2)
    (a : α) (b : β) : f (a, b) b = b := by
    simp [h]
  
  example {α β : Type} {f : α × β → β → β}
    (a : α) (b : β) (h : f (a,b) (a,b).2 = (a,b).2) : f (a, b) b = b := by
    simp [h]
  

  In both cases, h is applicable because simp does not index f-arguments anymore when adding h to the simp-set.
  It's important to note, however, that global theorems continue to be indexed in the usual manner.

• Improved the error messages produced by the decide tactic. #3422

• Improved auto-completion performance. #3460

• Improved initial language server startup performance. #3552

• Changed call hierarchy to sort entries and strip private header from names displayed in the call hierarchy. #3482

• There is now a low-level error recovery combinator in the parsing framework, primarily intended for DSLs. #3413

• You can now write termination_by? after a declaration to see the automatically inferred
  termination argument, and turn it into a termination_by … clause using the “Try this” widget or a code action. #3514

• A large fraction of Std has been moved into the Lean repository.
  This was motivated by:

  1. Making universally useful tactics such as ext, by_cases, change at,
     norm_cast, rcases, simpa, simp?, omega, and exact?
     available to all users of Lean, without imports.
  2. Minimizing the syntactic changes between plain Lean and Lean with import Std.
  3. Simplifying the development process for the basic data types
     Nat, Int, Fin (and variants such as UInt64), List, Array,
     and BitVec as we begin making the APIs and simp normal forms for these types
     more complete and consistent.
  4. Laying the groundwork for the Std roadmap, as a library focused on
     essential datatypes not provided by the core langauge (e.g. RBMap)
     and utilities such as basic IO.
     While we have achieved most of our initial aims in v4.7.0-rc1,
     some upstreaming will continue over the coming months.

• The / and % notations in Int now use Int.ediv and Int.emod
  (i.e. the rounding conventions have changed).
  Previously Std overrode these notations, so this is no change for users of Std.
  There is now kernel support for these functions.
  #3376.

• omega, our integer linear arithmetic tactic, is now availabe in the core langauge.

  • It is supplemented by a preprocessing tactic bv_omega which can solve goals about BitVec
    which naturally translate into linear arithmetic problems.
    #3435.
  • omega now has support for Fin #3427,
    the <<< operator #3433.
  • During the port omega was modified to no longer identify atoms up to definitional equality
    (so in particular it can no longer prove id x ≤ x). #3525.
    This may cause some regressions.
    We plan to provide a general purpose preprocessing tactic later, or an omega! mode.
  • omega is now invoked in Lean's automation for termination proofs
    #3503 as well as in
    array indexing proofs #3515.
    This automation will be substantially revised in the medium term,
    and while omega does help automate some proofs, we plan to make this much more robust.

• The library search tactics exact? and apply? that were originally in
  Mathlib are now available in Lean itself. These use the implementation using
  lazy discrimination trees from Std, and thus do not require a disk cache but
  have a slightly longer startup time. The order used for selection lemmas has
  changed as well to favor goals purely based on how many terms in the head
  pattern match the current goal.

• The solve_by_elim tactic has been ported from Std to Lean so that library
  search can use it.

• New #check_tactic and #check_simp commands have been added. These are
  useful for checking tactics (particularly simp) behave as expected in test
  suites.

• Previously, app unexpanders would only be applied to entire applications. However, some notations produce
  functions, and these functions can be given additional arguments. The solution so far has been to write app unexpanders so that they can take an arbitrary number of additional arguments. However this leads to misleading hover information in the Infoview. For example, while HAdd.hAdd f g 1 pretty prints as (f + g) 1, hovering over f + g shows f. There is no way to fix the situation from within an app unexpander; the expression position for HAdd.hAdd f g is absent, and app unexpanders cannot register TermInfo.

  This commit changes the app delaborator to try running app unexpanders on every prefix of an application, from longest to shortest prefix. For efficiency, it is careful to only try this when app delaborators do in fact exist for the head constant, and it also ensures arguments are only delaborated once. Then, in (f + g) 1, the f + g gets TermInfo registered for that subexpression, making it properly hoverable.

  #3375

Breaking changes:

• Lean.withTraceNode and variants got a stronger MonadAlwaysExcept assumption to
  fix trace trees not being built on elaboration runtime exceptions. Instances for most elaboration
  monads built on EIO Exception should be synthesized automatically.
• The match ... with. and fun. notations previously in Std have been replaced by
  nomatch ... and nofun. #3279 and #3286

Other improvements:

• several bug fixes for simp:
  • we should not crash when simp loops #3269
  • simp gets stuck on autoParam #3315
  • simp fails when custom discharger makes no progress #3317
  • simp fails to discharge autoParam premises even when it can reduce them to True #3314
  • simp? suggests generated equations lemma names, fixes #3547 #3573
• fixes for match expressions:
  • fix regression with builtin literals #3521...

v4.6.1

Changes since v4.6.0 (from RELEASES.md)

• Backport of #3552 fixing a performance regression
  in server startup.

v4.6.0

Changes since v4.5.0 (from RELEASES.md)

• Add custom simplification procedures (aka simprocs) to simp. Simprocs can be triggered by the simplifier on a specified term-pattern. Here is an small example:

  import Lean.Meta.Tactic.Simp.BuiltinSimprocs.Nat
  
  def foo (x : Nat) : Nat :=
    x + 10
  
  /--
  The `simproc` `reduceFoo` is invoked on terms that match the pattern `foo _`.
  -/
  simproc reduceFoo (foo _) :=
    /- A term of type `Expr → SimpM Step -/
    fun e => do
      /-
      The `Step` type has three constructors: `.done`, `.visit`, `.continue`.
      * The constructor `.done` instructs `simp` that the result does
        not need to be simplied further.
      * The constructor `.visit` instructs `simp` to visit the resulting expression.
      * The constructor `.continue` instructs `simp` to try other simplification procedures.
  
      All three constructors take a `Result`. The `.continue` contructor may also take `none`.
      `Result` has two fields `expr` (the new expression), and `proof?` (an optional proof).
       If the new expression is definitionally equal to the input one, then `proof?` can be omitted or set to `none`.
      -/
      /- `simp` uses matching modulo reducibility. So, we ensure the term is a `foo`-application. -/
      unless e.isAppOfArity ``foo 1 do
        return .continue
      /- `Nat.fromExpr?` tries to convert an expression into a `Nat` value -/
      let some n ← Nat.fromExpr? e.appArg!
        | return .continue
      return .done { expr := Lean.mkNatLit (n+10) }

  We disable simprocs support by using the command set_option simprocs false. This command is particularly useful when porting files to v4.6.0.
  Simprocs can be scoped, manually added to simp commands, and suppressed using -. They are also supported by simp?. simp only does not execute any simproc. Here are some examples for the simproc defined above.

  example : x + foo 2 = 12 + x := by
    set_option simprocs false in
      /- This `simp` command does not make progress since `simproc`s are disabled. -/
      fail_if_success simp
    simp_arith
  
  example : x + foo 2 = 12 + x := by
    /- `simp only` must not use the default simproc set. -/
    fail_if_success simp only
    simp_arith
  
  example : x + foo 2 = 12 + x := by
    /-
    `simp only` does not use the default simproc set,
    but we can provide simprocs as arguments. -/
    simp only [reduceFoo]
    simp_arith
  
  example : x + foo 2 = 12 + x := by
    /- We can use `-` to disable `simproc`s. -/
    fail_if_success simp [-reduceFoo]
    simp_arith

  The command register_simp_attr <id> now creates a simp and a simproc set with the name <id>. The following command instructs Lean to insert the reduceFoo simplification procedure into the set my_simp. If no set is specified, Lean uses the default simp set.

  simproc [my_simp] reduceFoo (foo _) := ...

• The syntax of the termination_by and decreasing_by termination hints is overhauled:

  • They are now placed directly after the function they apply to, instead of
    after the whole mutual block.
  • Therefore, the function name no longer has to be mentioned in the hint.
  • If the function has a where clause, the termination_by and
    decreasing_by for that function come before the where. The
    functions in the where clause can have their own termination hints, each
    following the corresponding definition.
  • The termination_by clause can only bind “extra parameters”, that are not
    already bound by the function header, but are bound in a lambda (:= fun x y z =>) or in patterns (| x, n + 1 => …). These extra parameters used to
    be understood as a suffix of the function parameters; now it is a prefix.

  Migration guide: In simple cases just remove the function name, and any
  variables already bound at the header.

   def foo : Nat → Nat → Nat := …
  -termination_by foo a b => a - b
  +termination_by a b => a - b

  or

   def foo : Nat → Nat → Nat := …
  -termination_by _ a b => a - b
  +termination_by a b => a - b

  If the parameters are bound in the function header (before the :), remove them as well:

   def foo (a b : Nat) : Nat := …
  -termination_by foo a b => a - b
  +termination_by a - b

  Else, if there are multiple extra parameters, make sure to refer to the right
  ones; the bound variables are interpreted from left to right, no longer from
  right to left:

   def foo : Nat → Nat → Nat → Nat
     | a, b, c => …
  -termination_by foo b c => b
  +termination_by a b => b

  In the case of a mutual block, place the termination arguments (without the
  function name) next to the function definition:

  -mutual
  -def foo : Nat → Nat → Nat := …
  -def bar : Nat → Nat := …
  -end
  -termination_by
  -  foo a b => a - b
  -  bar a => a
  +mutual
  +def foo : Nat → Nat → Nat := …
  +termination_by a b => a - b
  +def bar : Nat → Nat := …
  +termination_by a => a
  +end

  Similarly, if you have (mutual) recursion through where or let rec, the
  termination hints are now placed directly after the function they apply to:

  -def foo (a b : Nat) : Nat := …
  -  where bar (x : Nat) : Nat := …
  -termination_by
  -  foo a b => a - b
  -  bar x => x
  +def foo (a b : Nat) : Nat := …
  +termination_by a - b
  +  where
  +    bar (x : Nat) : Nat := …
  +    termination_by x
  
  -def foo (a b : Nat) : Nat :=
  -  let rec bar (x : Nat) :  Nat := …
  -  …
  -termination_by
  -  foo a b => a - b
  -  bar x => x
  +def foo (a b : Nat) : Nat :=
  +  let rec bar (x : Nat) : Nat := …
  +    termination_by x
  +  …
  +termination_by a - b

  In cases where a single decreasing_by clause applied to multiple mutually
  recursive functions before, the tactic now has to be duplicated.

• The semantics of decreasing_by changed; the tactic is applied to all
  termination proof goals together, not individually.

  This helps when writing termination proofs interactively, as one can focus
  each subgoal individually, for example using ·. Previously, the given
  tactic script had to work for all goals, and one had to resort to tactic
  combinators like first:

   def foo (n : Nat) := … foo e1 … foo e2 …
  -decreasing_by
  -simp_wf
  -first | apply something_about_e1; …
  -      | apply something_about_e2; …
  +decreasing_by
  +all_goals simp_wf
  +· apply something_about_e1; …
  +· apply something_about_e2; …

  To obtain the old behaviour of applying a tactic to each goal individually,
  use all_goals:

   def foo (n : Nat) := …
  -decreasing_by some_tactic
  +decreasing_by all_goals some_tactic

  In the case of mutual recursion each decreasing_by now applies to just its
  function. If some functions in a recursive group do not have their own
  decreasing_by, the default decreasing_tactic is used. If the same tactic
  ought to be applied to multiple functions, the decreasing_by clause has to
  be repeated at each of these functions.

• Modify InfoTree.context to facilitate augmenting it with partial contexts while elaborating a command. This breaks backwards compatibility with all downstream projects that traverse the InfoTree manually instead of going through the functions in InfoUtils.lean, as well as those manually creating and saving InfoTrees. See PR #3159 for how to migrate your code.

• Add language server support for call hierarchy requests (PR #3082). The change to the .ilean format in this PR means that projects must be fully rebuilt once in order to generate .ilean files with the new format before features like "find references" work correctly again.

• Structure instances with multiple sources (for example {a, b, c with x := 0}) now have their fields filled from these sources
  in strict left-to-right order. Furthermore, the structure instance elaborator now aggressively use sources to fill in subobject
  fields, which prevents unnecessary eta expansion of the sources,
  and hence greatly reduces the reliance on costly structure eta reduction. This has a large impact on mathlib,
  reducing total CPU instructions by 3% and enabling impactful refactors like leanprover-community/mathlib4#8386
  which reduces the build time by almost 20%.
  See PR #2478 and RFC #2451.

• Add pretty printer settings to omit deeply nested terms (pp.deepTerms false and pp.deepTerms.threshold) (PR #3201)

• Add pretty printer options pp.numeralTypes and pp.natLit.
  When pp.numeralTypes is true, then natural number literals, integer literals, and rational number literals
  are pretty printed with type ascriptions, such as (2 : Rat), (-2 : Rat), and (-2 / 3 : Rat).
  When pp.natLit is true, then raw natural number literals are pretty printed as nat_lit 2.
  PR #2933 and RFC #3021.

Lake updates:

• improved platform information & control #3226
• lake update from unsupported manifest versions #3149

Other improvements:

• make intro be aware of let_fun #3115
• produce simpler proof terms in rw #3121
• fuse nested `mkCongrArg...

v4.6.0-rc1

• Add custom simplification procedures (aka simprocs) to simp. Simprocs can be triggered by the simplifier on a specified term-pattern. Here is an small example:

import Lean.Meta.Tactic.Simp.BuiltinSimprocs.Nat

def foo (x : Nat) : Nat :=
  x + 10

/--
The `simproc` `reduceFoo` is invoked on terms that match the pattern `foo _`.
-/
simproc reduceFoo (foo _) :=
  /- A term of type `Expr → SimpM Step -/
  fun e => do
    /-
    The `Step` type has three constructors: `.done`, `.visit`, `.continue`.
    * The constructor `.done` instructs `simp` that the result does
      not need to be simplied further.
    * The constructor `.visit` instructs `simp` to visit the resulting expression.
    * The constructor `.continue` instructs `simp` to try other simplification procedures.

    All three constructors take a `Result`. The `.continue` contructor may also take `none`.
    `Result` has two fields `expr` (the new expression), and `proof?` (an optional proof).
     If the new expression is definitionally equal to the input one, then `proof?` can be omitted or set to `none`.
    -/
    /- `simp` uses matching modulo reducibility. So, we ensure the term is a `foo`-application. -/
    unless e.isAppOfArity ``foo 1 do
      return .continue
    /- `Nat.fromExpr?` tries to convert an expression into a `Nat` value -/
    let some n ← Nat.fromExpr? e.appArg!
      | return .continue
    return .done { expr := Lean.mkNatLit (n+10) }

We disable simprocs support by using the command set_option simprocs false. This command is particularly useful when porting files to v4.6.0.
Simprocs can be scoped, manually added to simp commands, and suppressed using -. They are also supported by simp?. simp only does not execute any simproc. Here are some examples for the simproc defined above.

example : x + foo 2 = 12 + x := by
  set_option simprocs false in
    /- This `simp` command does not make progress since `simproc`s are disabled. -/
    fail_if_success simp
  simp_arith

example : x + foo 2 = 12 + x := by
  /- `simp only` must not use the default simproc set. -/
  fail_if_success simp only
  simp_arith

example : x + foo 2 = 12 + x := by
  /-
  `simp only` does not use the default simproc set,
  but we can provide simprocs as arguments. -/
  simp only [reduceFoo]
  simp_arith

example : x + foo 2 = 12 + x := by
  /- We can use `-` to disable `simproc`s. -/
  fail_if_success simp [-reduceFoo]
  simp_arith

The command register_simp_attr <id> now creates a simp and a simproc set with the name <id>. The following command instructs Lean to insert the reduceFoo simplification procedure into the set my_simp. If no set is specified, Lean uses the default simp set.

simproc [my_simp] reduceFoo (foo _) := ...

• The syntax of the termination_by and decreasing_by termination hints is overhauled:

  • They are now placed directly after the function they apply to, instead of
    after the whole mutual block.
  • Therefore, the function name no longer has to be mentioned in the hint.
  • If the function has a where clause, the termination_by and
    decreasing_by for that function come before the where. The
    functions in the where clause can have their own termination hints, each
    following the corresponding definition.
  • The termination_by clause can only bind “extra parameters”, that are not
    already bound by the function header, but are bound in a lambda (:= fun x y z =>) or in patterns (| x, n + 1 => …). These extra parameters used to
    be understood as a suffix of the function parameters; now it is a prefix.

  Migration guide: In simple cases just remove the function name, and any
  variables already bound at the header.

   def foo : Nat → Nat → Nat := …
  -termination_by foo a b => a - b
  +termination_by a b => a - b

  or

   def foo : Nat → Nat → Nat := …
  -termination_by _ a b => a - b
  +termination_by a b => a - b

  If the parameters are bound in the function header (before the :), remove them as well:

   def foo (a b : Nat) : Nat := …
  -termination_by foo a b => a - b
  +termination_by a - b

  Else, if there are multiple extra parameters, make sure to refer to the right
  ones; the bound variables are interpreted from left to right, no longer from
  right to left:

   def foo : Nat → Nat → Nat → Nat
     | a, b, c => …
  -termination_by foo b c => b
  +termination_by a b => b

  In the case of a mutual block, place the termination arguments (without the
  function name) next to the function definition:

  -mutual
  -def foo : Nat → Nat → Nat := …
  -def bar : Nat → Nat := …
  -end
  -termination_by
  -  foo a b => a - b
  -  bar a => a
  +mutual
  +def foo : Nat → Nat → Nat := …
  +termination_by a b => a - b
  +def bar : Nat → Nat := …
  +termination_by a => a
  +end

  Similarly, if you have (mutual) recursion through where or let rec, the
  termination hints are now placed directly after the function they apply to:

  -def foo (a b : Nat) : Nat := …
  -  where bar (x : Nat) : Nat := …
  -termination_by
  -  foo a b => a - b
  -  bar x => x
  +def foo (a b : Nat) : Nat := …
  +termination_by a - b
  +  where
  +    bar (x : Nat) : Nat := …
  +    termination_by x
  
  -def foo (a b : Nat) : Nat :=
  -  let rec bar (x : Nat) :  Nat := …
  -  …
  -termination_by
  -  foo a b => a - b
  -  bar x => x
  +def foo (a b : Nat) : Nat :=
  +  let rec bar (x : Nat) : Nat := …
  +    termination_by x
  +  …
  +termination_by a - b

  In cases where a single decreasing_by clause applied to multiple mutually
  recursive functions before, the tactic now has to be duplicated.

• The semantics of decreasing_by changed; the tactic is applied to all
  termination proof goals together, not individually.

  This helps when writing termination proofs interactively, as one can focus
  each subgoal individually, for example using ·. Previously, the given
  tactic script had to work for all goals, and one had to resort to tactic
  combinators like first:

   def foo (n : Nat) := … foo e1 … foo e2 …
  -decreasing_by
  -simp_wf
  -first | apply something_about_e1; …
  -      | apply something_about_e2; …
  +decreasing_by
  +all_goals simp_wf
  +· apply something_about_e1; …
  +· apply something_about_e2; …

  To obtain the old behaviour of applying a tactic to each goal individually,
  use all_goals:

   def foo (n : Nat) := …
  -decreasing_by some_tactic
  +decreasing_by all_goals some_tactic

  In the case of mutual recursion each decreasing_by now applies to just its
  function. If some functions in a recursive group do not have their own
  decreasing_by, the default decreasing_tactic is used. If the same tactic
  ought to be applied to multiple functions, the decreasing_by clause has to
  be repeated at each of these functions.

• Modify InfoTree.context to facilitate augmenting it with partial contexts while elaborating a command. This breaks backwards compatibility with all downstream projects that traverse the InfoTree manually instead of going through the functions in InfoUtils.lean, as well as those manually creating and saving InfoTrees. See PR #3159 for how to migrate your code.

• Add language server support for call hierarchy requests (PR #3082). The change to the .ilean format in this PR means that projects must be fully rebuilt once in order to generate .ilean files with the new format before features like "find references" work correctly again.

• Structure instances with multiple sources (for example {a, b, c with x := 0}) now have their fields filled from these sources
  in strict left-to-right order. Furthermore, the structure instance elaborator now aggressively use sources to fill in subobject
  fields, which prevents unnecessary eta expansion of the sources,
  and hence greatly reduces the reliance on costly structure eta reduction. This has a large impact on mathlib,
  reducing total CPU instructions by 3% and enabling impactful refactors like leanprover-community/mathlib4#8386
  which reduces the build time by almost 20%.
  See PR #2478 and RFC #2451.

• Add pretty printer settings to omit deeply nested terms (pp.deepTerms false and pp.deepTerms.threshold) (PR #3201)

• Add pretty printer options pp.numeralTypes and pp.natLit.
  When pp.numeralTypes is true, then natural number literals, integer literals, and rational number literals
  are pretty printed with type ascriptions, such as (2 : Rat), (-2 : Rat), and (-2 / 3 : Rat).
  When pp.natLit is true, then raw natural number literals are pretty printed as nat_lit 2.
  PR #2933 and RFC #3021.

Lake updates:

• improved platform information & control #3226
• lake update from unsupported manifest versions #3149

Other improvements:

• make intro be aware of let_fun #3115
• produce simpler proof terms in rw #3121
• fuse nested mkCongrArg calls in proofs generated by simp #3203
• induction using followed by a general term [#3188](https://github....