Crate lemma 

Lemma Engine

Rules for man and machine

Lemma is a declarative programming language for expressing rules, data, and business logic in a way that is both human-readable and machine-executable.

Quick Start

use lemma::{Engine, SourceType};

let mut engine = Engine::new();

// Load Lemma code
engine.load(r#"
    spec example
    data price: 100
    data quantity: 5
    rule total: price * quantity
"#, SourceType::Path(std::sync::Arc::new(std::path::PathBuf::from("example.lemma")))).expect("failed to load");

// Evaluate the spec (all rules, no data values)
let now = lemma::DateTimeValue::now();
let response = engine.run(None, "example", Some(&now), std::collections::HashMap::new(), false).unwrap();

Core Concepts

Specs

A spec is a collection of data and rules. Specs can reference other Specs to build composable logic.

Data

Data are named values: numbers, text, dates, booleans, or typed units like 50 kilograms or 100.

Rules

Rules compute values based on data and other rules. They support conditional logic through “unless” clauses.

Types

Lemma has a rich type system including units (mass, length, time, money) with automatic conversions.

Modules

deps

Dependency layout under <workdir>/lemma_deps/, shared by CLI fetch and LSP.

Structs

Context

Ordered store of specs keyed by (repository, name) and grouped into LemmaSpecSets.

ConversionTraceStep

DataEntry

A spec’s public interface: its data (inputs) and rules (outputs) with full structured type information.

DataGroup

Grouped data from a specific spec (semantics types only).

DataPath

Resolved path to a data (created during planning from AST DataReference)

DateTimeValue

Engine

Engine for evaluating Lemma rules.

ErrorDetails

Detailed error information with optional source location.

Errors

Load failure: errors plus the source texts we attempted to load.

EvaluationTrace

ExecutionPlan

A complete execution plan ready for the evaluator

ExecutionPlanSerialized

The serializable form of an ExecutionPlan.

InversionResponse

Response from inversion containing all valid solutions

LemmaBase

The LemmaBase registry fetches Lemma source text from LemmaBase.

LemmaRepository

A Lemma repository header. Identity carrier; never owns specs.

LemmaSpec

A Lemma spec containing data and rules.

LemmaSpecSet

All spec versions sharing a (repository, name) identity, keyed by effective_from.

LemmaType

Resolved type after planning

Lexer

LiteralValue

Literal value with type. The single value type in semantics.

ParseResult

PlanningSource

Positional source location: source id and span.

QuantityUnit

A single unit within a Quantity type.

QuantityUnits

RatioUnit

RatioUnits

RegistryBundle

A bundle of Lemma source text returned by the Registry.

RegistryError

An error returned by a Registry implementation.

ResolvedRepository

A loaded repository with all its spec sets.

ResourceLimits

Limits to prevent abuse and enable predictable resource usage

Response

Response from evaluating a Lemma spec

RulePath

Resolved path to a rule (created during planning from RuleReference)

RuleResult

Result of evaluating a single rule. Struct fields match the API JSON shape.

Source

Positional source location: source id and span.

Span

Span representing a location in source code

SpecRef

A spec reference written in source.

SpecSchema

Target

Desired outcome for an inversion query

TimezoneValue

TraceBranch

TraceNonMatchedBranch

Enums

BindingDataValue

Response/UI row for spec data: LemmaType plus optional bound literal (mirrors parse-time Definition).

Bound

Bound specification for ranges

ComputationKind

The kind of computation performed

ConversionTraceRole

DataDefinition

Resolved data value for the execution plan: aligned with DataValue but with source per variant.

DataValue

Parse-time data value (before type resolution)

DataValueInput

Typed data value from a client (CLI/WASM). JSON parsing stays outside [parse_data_value].

Domain

Domain specification for valid values

EffectiveDate

Error

Error types for the Lemma system with source location tracking

ErrorKind

Classification of an Error. Serialized as the kind field on the flat object returned to JavaScript from WASM (engine/src/wasm.rs, JsError).

OperationResult

Result of an operation (evaluating a rule or expression)

RegistryErrorKind

The kind of failure that occurred during a Registry operation.

RequestErrorKind

Distinguishes HTTP 404 (not found) from 400 (bad request) for request errors.

SourceType

TargetOp

Comparison operators for targets

TokenKind

TraceNode

TraceValueSource

TypeSpecification

ValueKind

Value payload (shape of a literal). No type attached. Quantity unit is required; Ratio unit is optional (see plan ratio-units-optional.md).

VetoType

Why an operation yielded no value (domain veto).

Constants

EMBEDDED_STDLIB_REPOSITORY

Repository name reserved for the embedded standard library (repo lemma, spec units). User Engine::load / Engine::load_batch must not target this name.

MAX_DATA_NAME_LENGTH

MAX_RULE_NAME_LENGTH

MAX_SPEC_NAME_LENGTH

UNITS_LEMMA

Traits

Registry

Trait for resolving external repository references.

Functions

collect_lemma_sources

Collect .lemma source texts from filesystem paths (paths and one-level directories). Does not touch an Engine; pair with Engine::load / Engine::load_batch.

format_parse_result

Format a ParseResult (repository groups + specs) into canonical Lemma source.

format_source

Parse a source string and format it to canonical Lemma source.

format_specs

Format a sequence of parsed specs into canonical Lemma source.

parse

parse_spec_set_id

Validate and normalize a spec set identifier (a workspace spec name).

plan

Build execution plans for one or more Lemma specs.

resolve_registry_references

Resolve every uses reference that carries a registry repository qualifier in the loaded specs.

trace_expression

Human-readable expression text for a trace node used when composing parent expressions.

type_detail_lines

Produce a human-readable summary of type constraints, or None when there are no constraints worth showing (e.g. bare boolean). Returns one formatted string per constraint or property of the type specification. Uses rational_to_display_str for all rational bounds so they render as decimals, not as raw fractions.