lambda-tool-python

Python bindings for λ-Tool: safe LLM tool composition with type checking.

λ-Tool is a typed intermediate representation that LLMs generate instead of Python for tool composition. The type checker verifies code is safe before execution -- no missing fields, no unhandled errors, no infinite loops.

Prerequisites

• Python 3.10+
• The lambda_tool CLI binary (build from source)

# Build the OCaml CLI
cd lambda-tool
opam install . --deps-only && eval $(opam env)
dune build && dune install

Install

pip install lambda-tool

The package auto-discovers the lambda_tool binary from PATH, dune build output, or opam install locations.

Quick Start

from lambda_tool import LambdaTool, LambdaToolError

lt = LambdaTool()

# Type check a program
result = lt.typecheck("let x = 1 + 2 in x")
print(result.type)     # "Int"
print(result.effects)  # []

# Execute with real tool implementations
result = lt.run("""
tool query: String -{Read}-> {rows: List {id: Int, name: String}};

let get_name = fn row: {id: Int, name: String} => row.name in
match exec tool query "SELECT * FROM users" {
  Ok(result) => map get_name result.rows,
  Err(e) => []: String
}
""", executors={
    "query": lambda arg: {"rows": [{"id": 1, "name": "Alice"}]},
})

print(result.value)    # ["Alice"]
print(result.type)     # "List String"
print(result.effects)  # ["Read"]

Error Handling

Type errors are caught before execution:

try:
    lt.typecheck("let x = 1 + true in x")
except LambdaToolError as e:
    print(e.errors)  # ["Type mismatch at line 1, col 9: expected Bool, got Int"]

Runtime tool errors are caught and converted to Err values:

def flaky_tool(arg):
    raise ConnectionError("network down")

result = lt.run("""
tool fetch: String -{Network}-> String;
match exec tool fetch "url" {
  Ok(data) => data,
  Err(e) => "fallback"
}
""", executors={"fetch": flaky_tool})

print(result.value)  # "fallback"

API Reference

LambdaTool(binary=None)

Create a wrapper instance. Pass binary to override auto-discovery of the lambda_tool CLI.

lt.typecheck(source) -> TypeCheckResult

Type check without executing. Returns TypeCheckResult with:

• .type (str) -- the inferred type
• .effects (list[str]) -- the required effects ("Read", "Write", "Network")

Raises LambdaToolError on parse or type errors.

lt.run(source, executors=None) -> RunResult

Type check and execute. Returns RunResult with:

• .value -- the program's return value (decoded to Python)
• .type (str) -- the inferred type
• .effects (list[str]) -- the required effects

Raises LambdaToolError on parse, type, or runtime errors.

Executor Format

Each executor is a Python callable (arg) -> result:

Value Decoding

Testing

pytest tests/ -v

17 tests covering type checking, execution, interactive tool callbacks, traverse, conditionals, and edge cases.

Related Projects

• lambda-tool: The core OCaml implementation (type checker, interpreter, CLI)
• minilambda: Minimal agent demo using Claude + λ-Tool

License

LGPL-3.0-or-later. Check LICENSE for details. © Sarthak Shah (matchcase), 2026.