Pantograph

A Machine-to-Machine interaction system for Lean 4.

Pantograph provides interfaces to execute proofs, construct expressions, and examine the symbol list of a Lean project for machine learning.

Installation

For Nix based workflow, see below.

Install elan and lake, and run

lake build

This builds the executable in .lake/build/bin/pantograph.

To use Pantograph in a project environment, setup the LEAN_PATH environment variable so it contains the library path of lean libraries. The libraries must be built in advance. For example, if mathlib4 is stored at ../lib/mathlib4, the environment might be setup like this:

LIB="../lib"
LIB_MATHLIB="$LIB/mathlib4/lake-packages"
export LEAN_PATH="$LIB/mathlib4/build/lib:$LIB_MATHLIB/aesop/build/lib:$LIB_MATHLIB/Qq/build/lib:$LIB_MATHLIB/std/build/lib"

LEAN_PATH=$LEAN_PATH build/bin/pantograph $@

The $LEAN_PATH executable of any project can be extracted by

lake env printenv LEAN_PATH

Executable Usage

pantograph MODULES|LEAN_OPTIONS

The REPL loop accepts commands as single-line JSON inputs and outputs either an Error: (indicating malformed command) or a JSON return value indicating the result of a command execution. The command can be passed in one of two formats

command { ... }
{ "cmd": command, "payload": ... }

The list of available commands can be found in Pantograph/Protocol.lean and below. An empty command aborts the REPL.

The pantograph executable must be run with a list of modules to import. It can also accept lean options of the form --key=value e.g. --pp.raw=true.

Example: (~5k symbols)

$ pantograph Init
env.catalog
env.inspect {"name": "Nat.le_add_left"}

Example with mathlib4 (~90k symbols, may stack overflow, see troubleshooting)

$ pantograph Mathlib.Analysis.Seminorm
env.catalog

Example proving a theorem: (alternatively use goal.start {"copyFrom": "Nat.add_comm"}) to prime the proof

$ pantograph Init
goal.start {"expr": "∀ (n m : Nat), n + m = m + n"}
goal.tactic {"stateId": 0, "goalId": 0, "tactic": "intro n m"}
goal.tactic {"stateId": 1, "goalId": 0, "tactic": "assumption"}
goal.delete {"stateIds": [0]}
stat {}
goal.tactic {"stateId": 1, "goalId": 0, "tactic": "rw [Nat.add_comm]"}
stat

where the application of assumption should lead to a failure.

Commands

See Pantograph/Protocol.lean for a description of the parameters and return values in JSON.

• reset: Delete all cached expressions and proof trees
• stat: Display resource usage
• expr.echo {"expr": <expr>, "type": <optional expected type>, ["levels": [<levels>]]}: Determine the type of an expression and format it.
• env.catalog: Display a list of all safe Lean symbols in the current environment
• env.inspect {"name": <name>, "value": <bool>}: Show the type and package of a given symbol; If value flag is set, the value is printed or hidden. By default only the values of definitions are printed.
• options.set { key: value, ... }: Set one or more options (not Lean options; those have to be set via command line arguments.), for options, see Pantograph/Protocol.lean
• options.print: Display the current set of options
• goal.start {["name": <name>], ["expr": <expr>], ["levels": [<levels>]], ["copyFrom": <symbol>]}: Start a new proof from a given expression or symbol
• goal.tactic {"stateId": <id>, "goalId": <id>, ...}: Execute a tactic string on a given goal. The tactic is supplied as additional key-value pairs in one of the following formats:
  • { "tactic": <tactic> }: Execute an ordinary tactic
  • { "expr": <expr> }: Assign the given proof term to the current goal
  • { "have": <expr>, "binderName": <name> }: Execute have and creates a branch goal
  • { "calc": <expr> }: Execute one step of a calc tactic. Each step must be of the form lhs op rhs. An lhs of _ indicates that it should be set to the previous rhs.
  • { "conv": <bool> }: Enter or exit conversion tactic mode. In the case of exit, the goal id is ignored.
• goal.continue {"stateId": <id>, ["branch": <id>], ["goals": <names>]}: Execute continuation/resumption
  • { "branch": <id> }: Continue on branch state. The current state must have no goals.
  • { "goals": <names> }: Resume the given goals
• goal.remove {"stateIds": [<id>]}": Drop the goal states specified in the list
• goal.print {"stateId": <id>}": Print a goal state

Errors

When an error pertaining to the execution of a command happens, the returning JSON structure is

{ "error": "type", "desc": "description" }

Common error forms:

• command: Indicates malformed command structure which results from either invalid command or a malformed JSON structure that cannot be fed to an individual command.
• index: Indicates an invariant maintained by the output of one command and input of another is broken. For example, attempting to query a symbol not existing in the library or indexing into a non-existent proof state.

Troubleshooting

If lean encounters stack overflow problems when printing catalog, execute this before running lean:

ulimit -s unlimited

Library Usage

Pantograph/Library.lean exposes a series of interfaces which allow FFI call with Pantograph which mirrors the REPL commands above. It is recommended to call Pantograph via this FFI since it provides a tremendous speed up.

Developing

A Lean development shell is provided in the Nix flake.

Testing

The tests are based on LSpec. To run tests,

lake test

You can run an individual test by specifying a prefix

lake test -- "Tactic/No Confuse"

Nix based workflow

The included Nix flake provides build targets for sharedLib and executable. The executable can be used as-is, but linking against the shared library requires the presence of lean-all.

To run tests:

nix flake check