v4.9.0

Language features, tactics, and metaprograms

• Definition transparency
  • #4053 adds the seal and unseal commands, which make definitions locally be irreducible or semireducible.
  • #4061 marks functions defined by well-founded recursion with @[irreducible] by default,
    which should prevent the expensive and often unfruitful unfolding of such definitions (see breaking changes below).
• Incrementality
  • #3940 extends incremental elaboration into various steps inside of declarations:
    definition headers, bodies, and tactics.
    .
  • 250994 and 67338b add @[incremental] attribute to mark an elaborator as supporting incremental elaboration.
  • #4259 improves resilience by ensuring incremental commands and tactics are reached only in supported ways.
  • #4268 adds special handling for := by so that stray tokens in tactic blocks do not inhibit incrementality.
  • #4308 adds incremental have tactic.
  • #4340 fixes incorrect info tree reuse.
  • #4364 adds incrementality for careful command macros such as set_option in theorem, theorem foo.bar, and lemma.
  • #4395 adds conservative fix for whitespace handling to avoid incremental reuse leading to goals in front of the text cursor being shown.
  • #4407 fixes non-incremental commands in macros blocking further incremental reporting.
  • #4436 fixes incremental reporting when there are nested tactics in terms.
• Functional induction
  • #4135 ensures that the names used for functional induction are reserved.
  • #4327 adds support for structural recursion on reflexive types.
    For example,

    inductive Many (α : Type u) where
      | none : Many α
      | more : α → (Unit → Many α) → Many α
    
    def Many.map {α β : Type u} (f : α → β) : Many α → Many β
      | .none => .none
      | .more x xs => .more (f x) (fun _ => (xs ()).map f)
    
    #check Many.map.induct
    /-
    Many.map.induct {α β : Type u} (f : α → β) (motive : Many α → Prop)
      (case1 : motive Many.none)
      (case2 : ∀ (x : α) (xs : Unit → Many α), motive (xs ()) → motive (Many.more x xs)) :
      ∀ (a : Many α), motive a
    -/
    

• #3903 makes the Lean frontend normalize all line endings to LF before processing.
  This lets Lean be insensitive to CRLF vs LF line endings, improving the cross-platform experience and making Lake hashes be faithful to what Lean processes.
• #4130 makes the tactic framework be able to recover from runtime errors (for example, deterministic timeouts or maximum recursion depth errors).
• split tactic
  • #4211 fixes split at h when h has forward dependencies.
  • #4349 allows split for if-expressions to work on non-propositional goals.
• apply tactic
  • #3929 makes error message for apply show implicit arguments in unification errors as needed.
    Modifies MessageData type (see breaking changes below).
• cases tactic
  • #4224 adds support for unification of offsets such as x + 20000 = 20001 in cases tactic.
• omega tactic
  • #4073 lets omega fall back to using classical Decidable instances when setting up contradiction proofs.
  • #4141 and #4184 fix bugs.
  • #4264 improves omega error message if no facts found in local context.
  • #4358 improves expression matching in omega by using match_expr.
• simp tactic

  • #4176 makes names of erased lemmas clickable.

  • #4208 adds a pretty printer for discrimination tree keys.

  • #4202 adds Simp.Config.index configuration option,
    which controls whether to use the full discrimination tree when selecting candidate simp lemmas.
    When index := false, only the head function is taken into account, like in Lean 3.
    This feature can help users diagnose tricky simp failures or issues in code from libraries
    developed using Lean 3 and then ported to Lean 4.

    In the following example, it will report that foo is a problematic theorem.

    opaque f : Nat → Nat → Nat
    
    @[simp] theorem foo : f x (x, y).2 = y := by sorry
    
    example : f a b ≤ b := by
      set_option diagnostics true in
      simp (config := { index := false })
    /-
    [simp] theorems with bad keys
      foo, key: f _ (@Prod.mk ℕ ℕ _ _).2
    -/

    With the information above, users can annotate theorems such as foo using no_index for problematic subterms. Example:

    opaque f : Nat → Nat → Nat
    
    @[simp] theorem foo : f x (no_index (x, y).2) = y := by sorry
    
    example : f a b ≤ b := by
      simp -- `foo` is still applied with `index := true`

  • #4274 prevents internal match equational theorems from appearing in simp trace.

  • #4177 and #4359 make simp continue even if a simp lemma does not elaborate, if the tactic state is in recovery mode.

  • #4341 fixes panic when applying @[simp] to malformed theorem syntax.

  • #4345 fixes simp so that it does not use the forward version of a user-specified backward theorem.

  • #4352 adds missing dsimp simplifications for fixed parameters of generated congruence theorems.

  • #4362 improves trace messages for simp so that constants are hoverable.

• Elaboration
  • #4046 makes subst notation (he ▸ h) try rewriting in both directions even when there is no expected type available.
  • #3328 adds support for identifiers in autoparams (for example, rfl in (h : x = y := by exact rfl)).
  • #4096 changes how the type in let and have is elaborated, requiring that any tactics in the type be evaluated before proceeding, improving performance.
  • #4215 ensures the expression tree elaborator commits to the computed "max type" for the entire arithmetic expression.
  • #4267 cases signature elaboration errors to show even if there are parse errors in the body.
  • #4368 improves error messages when numeric literals fail to synthesize an OfNat instance,
    including special messages warning when the expected type of the numeral can be a proposition.
• Metaprogramming
  • #4167 adds Lean.MVarId.revertAll to revert all free variables.
  • #4169 adds Lean.MVarId.ensureNoMVar to ensure the goal's target contains no expression metavariables.
  • #4180 adds cleanupAnnotations parameter to forallTelescope methods.
  • #4307 adds support for parser aliases in syntax quotations.
• Work toward implementing grind tactic
  • 0a515e and #4164 add grind_norm and grind_norm_proc attributes and @[grind_norm] theorems.
  • #4170, #4221, and #4249 create grind preprocessor and core module.
  • #4235 and d6709e add special cases tactic to grind along with @[grind_cases] attribute to mark types that this cases tactic should automatically apply to.
  • #4243 adds special injection? tactic to grind.
• Other fixes or improvements
  • #4065 fixes a bug in the Nat.reduceLeDiff simproc.
  • #3969 makes deprecation warnings activate even for generalized field notation ("dot notation").
  • #4132 fixes the sorry term so that it does not activate the implicit lambda feature
  • 9803c5 and 47c8e3 move cdot and calc parsers to Lean namespace.
  • #4252 fixes the case tactic so that it is usable in macros by having it erase macro scopes from the tag.
  • 26b671 and cc33c3 extract haveId syntax.
  • #4335 fixes bugs in partial calc tactic when there is mdata or metavariables.
  • #4329 makes termination_by? report unused each unused parameter as _.
• Docs: #4238, #4294, #4338.

Language server, widgets, and IDE extensions

• #4066 fixes features like "Find References" when browsing core Lean sources.
• #4254 allows embedding user widgets in structured messages.
  Companion PR is vscode-lean4#449.
• #4445 makes watchdog more resilient against badly behaving clients.

Library

• #4059 upstreams many List and Array operations and theorems from Batteries.
• #4055 removes the unused Inhabited instance for Subtype.
• #3967 adds dates in existing @[deprecated] attributes.
• #4231 adds boilerplate Char, UInt, and Fin theorems.
• #4205 fixes the MonadStore type classes to use semiOutParam.
• #4350 renames IsLawfulSingleton to LawfulSingleton.
• Nat
  • #4094 swaps Nat.zero_or and Nat.or_zero.
  • #4098 and #4145 change the definition of Nat.mod so that n % (m + n) reduces when n is literal without relying on well-founded recursion, which becomes irreducible by default in #4061.
  • #4188 redefines Nat.testBit to be more performant.
  • Theorems: #4199.
• Array
  • #4074 improves the functional induction principle Array.feraseIdx.induct.
• List
  • #4172 removes @[simp] from List.length_pos.
• Option
  • #4037 adds theorems to simplify Option-valued dependent if-then-else.
  • #4314 removes @[simp] from Option.bind_eq_some.
• BitVec
  • Theorems: #3920, #4095, #4075, #4148, #4165, #4178, #4200, #4201, #4298, #4299, #4257, #4179, #4321, #4187.
  • #4193 adds simprocs for reducing x >>> i and x <<< i where i is a bitvector literal.
  • #4194 adds simprocs for reducing (x <<< i) <<< j and (x >>> i) >>> j where i and j are natural number literals.
  • #4229 redefines rotateLeft/rotateRight to use modulo reduction of shift offset.
  • 0d3051 makes <num>#<term> bitvector literal notation global.
• Char/String
  • #4143 modifies String.substrEq to avoid linter warnings in downstream code.
  • #4233 adds simprocs for Char and String inequalities.
  • #4348 upstreams Mathlib lemmas.
  • #4354 upstreams basic String lemmas.
• HashMap
  • #4248 fixes implicitness of typeclass arguments in HashMap.ofList.
• IO
  • #4036 adds IO.Process.getCurrentDir and IO.Process.setCurrentDir for adjusting the current process's working directory.
• Cleanup: #4077, #4189, #4304.
• Docs: #4001, #4166, #4332.

Lean internals

• Defeq and WHNF algorithms
  • #4029 remove unnecessary checkpointDefEq
  • #4206 fixes isReadOnlyOrSyntheticOpaque to respect metavariable depth.
  • #4217 fixes missing occurs check for delayed assignments.
• Definition transparency
  • #4052 adds validation to application of @[reducible]/@[semireducible]/@[irreducible] attributes (with local/scoped modifiers as well).
    Setting set_option allowUnsafeReductibility true turns this validation off.
• Inductive types
  • #3591 fixes a bug where indices could be incorrectly promoted to parameters.
  • #3398 fixes a bug in the injectivity theorem generator.
  • #4342 fixes elaboration of mutual inductives with instance parameters.
• Diagnostics and profiling
  • #3986 adds option trace.profiler.useHeartbeats to switch trace.profiler.threshold to being in terms of heartbeats instead of milliseconds.
  • #4082 makes set_option diagnostics true report kernel diagnostic information.
• Typeclass resolution
  • #4119 fixes multiple issues with TC caching interacting with synthPendingDepth, adds maxSynthPendingDepth option with default value 1.
  • #4210 ensures local instance cache does not contain multiple copies of the same instance.
  • #4216 fix handling of metavariables, to avoid needing to set the option backward.synthInstance.canonInstances to false.
• Other fixes or improvements
  • #4080 fixes propagation of state for Lean.Elab.Command.liftCoreM and Lean.Elab.Command.liftTermElabM.
  • #3944 makes the Repr deriving handler be consistent between structure and inductive for how types and proofs are erased.
  • #4113 propagates maxHeartbeats to kernel to control "(kernel) deterministic timeout" error.
  • #4125 reverts #3970 (monadic generalization of FindExpr).
  • #4128 catches stack overflow in auto-bound implicits feature.
  • #4129 adds tryCatchRuntimeEx combinator to replace catchRuntimeEx reader state.
  • #4155 simplifies the expression canonicalizer.
  • #4151 and #4369
    add many missing trace classes.
  • #4185 makes congruence theorem generators clean up type annotations of argument types.
  • #4192 fixes restoration of infotrees when auto-bound implicit feature is activated,
    fixing a pretty printing error in hovers and strengthening the unused variable linter.
  • dfb496 fixes declareBuiltin to allow it to be called multiple times per declaration.
  • Cleanup: #4112, #4126, #4091, #4139, #4153.
  • Tests: 030406, #4133.

Compiler, runtime, and FFI

• #4100 improves reset/reuse algorithm; it now runs a second pass relaxing the constraint that reused memory cells must only be for the exact same constructor.
• #2903 fixes segfault in old compiler from mishandling noConfusion applications.
• #4311 fixes bug in constant folding.
• #3915 documents the runtime memory layout for inductive types.

Lake

• #4057 adds support for docstrings on require commands.
• #4088 improves hovers for family_def and library_data commands.
• #4147 adds default README.md to package templates
• #4261 extends lake test help page, adds help page for lake check-test,
  adds lake lint and tag @[lint_driver], adds support for specifying test and lint drivers from dependencies,
  adds testDriverArgs and lintDriverArgs options, adds support for library test drivers,
  makes lake check-test and lake check-lint only load the package without dependencies.
• #4270 adds lake pack and lake unpack for packing and unpacking Lake build artifacts from an archive.
• #4083 Switches the manifest format to use major.minor.patch semantic
  versions. Major version increments indicate breaking changes (e.g., new
  required fields and semantic changes to existing fields). Minor version
  increments (after 0.x) indicate backwards-compatible extensions (e.g.,
  adding optional fields, removing fields). This change is backwards
  compatible. Lake will still successfully read old manifests with numeric
  versions. It will treat the numeric version N as semantic version
  0.N.0. Lake will also accept manifest versions with - suffixes
  (e.g., x.y.z-foo) and then ignore the suffix.
• #4273 adds a lift from JobM to FetchM for backwards compatibility reasons.
• #4351 fixes LogIO-to-CliM-lifting performance issues.
• #4343 make Lake store the dependency trace for a build in
  the cached build long and then verifies that it matches the trace of the current build before replaying the log.
• #4402 moves the cached log into the trace file (no more .log.json).
  This means logs are no longer cached on fatal errors and this ensures that an out-of-date log is not associated with an up-to-date trace.
  Separately, .hash file generation was changed to be more reliable as well.
  The .hash files are deleted as part of the build and always regenerate with --rehash.
• Other fixes or improvements
  • #4056 cleans up tests
  • #4244 fixes noRelease test when Lean repo is tagged
  • #4346 improves tests/serve
  • #4356 adds build log path to the warning for a missing or invalid build log.

DevOps

• #3984 adds a script (script/rebase-stage0.sh) for git rebase -i that automatically updates each stage0.
• #4108 finishes renamings from transition to Std to Batteries.
• #4109 adjusts the Github bug template to mention testing using live.lean-lang.org.
• #4136 makes CI rerun only when full-ci label is added or removed.
• #4175 and 72b345 switch to using #guard_msgs to run tests as much as possible.
• #3125 explains the Lean4 pygments lexer.
• #4247 sets up a procedure for preparing release notes.
• #4032 modernizes build instructions and workflows.
• #4255 moves some expensive checks from merge queue to releases.
• #4265 adds aarch64 macOS as native compilation target for CI.
• f05a82 restores macOS aarch64 install suffix in CI
• #4317 updates build instructions for macOS.
• #4333 adjusts workflow to update Batteries in manifest when creating lean-pr-testing-NNNN Mathlib branches.
• #4355 simplifies lean4checker step of release checklist.
• #4361 adds installing elan to pr-release CI step.

Breaking changes

While most changes could be considered to be a breaking change, this section makes special note of API changes.

• Nat.zero_or and Nat.or_zero have been swapped (#4094).
• IsLawfulSingleton is now LawfulSingleton (#4350).
• BitVec.rotateLeft and BitVec.rotateRight now take the shift modulo the bitwidth (#4229).
• These are no longer simp lemmas:
  List.length_pos (#4172),
  Option.bind_eq_some (#4314).
• Types in let and have (both the expressions and tactics) may fail to elaborate due to new restrictions on what sorts of elaboration problems may be postponed (#4096).
  In particular, tactics embedded in the type will no longer make use of the type of value in expressions such as let x : type := value; body.
• Now functions defined by well-founded recursion are marked with @[irreducible] by default (#4061).
  Existing proofs that hold by definitional equality (e.g. rfl) can be
  rewritten to explictly unfold the function definition (using simp,
  unfold, rw), or the recursive function can be temporarily made
  semireducible (using unseal f in before the command), or the function
  definition itself can be marked as @[semireducible] to get the previous
  behavior.
• Due to #3929:

  • The MessageData.ofPPFormat constructor has been removed.
    Its functionality has been split into two:

    • for lazy structured messages, please use MessageData.lazy;
    • for embedding Format or FormatWithInfos, use MessageData.ofFormatWithInfos.

    An example migration can be found in #3929.

  • The MessageData.ofFormat constructor has been turned into a function.
    If you need to inspect MessageData, you can pattern-match on MessageData.ofFormatWithInfos.