RodarFeel

A standalone FEEL (Friendly Enough Expression Language) evaluator for Elixir. FEEL is the standard expression language for BPMN 2.0 and DMN.

Extracted from the Rodar BPMN engine for independent use.

Installation

Add rodar_feel to your list of dependencies in mix.exs:

def deps do
  [{:rodar_feel, "~> 0.1.0"}]
end

Usage

# Simple arithmetic
RodarFeel.eval("1 + 2", %{})
# => {:ok, 3}

# Variable bindings
RodarFeel.eval("amount > 1000", %{"amount" => 1500})
# => {:ok, true}

# If-then-else
RodarFeel.eval(~S(if x > 10 then "high" else "low"), %{"x" => 15})
# => {:ok, "high"}

# Built-in functions
RodarFeel.eval("string length(name)", %{"name" => "Alice"})
# => {:ok, 5}

# List and range membership
RodarFeel.eval(~S(status in ["active", "pending"]), %{"status" => "active"})
# => {:ok, true}

RodarFeel.eval("age in 18..65", %{"age" => 30})
# => {:ok, true}

# Null handling
RodarFeel.eval("missing + 5", %{})
# => {:ok, nil}

# Temporal types
RodarFeel.eval(~S(@"2024-03-20" + @"P10D"), %{})
# => {:ok, ~D[2024-03-30]}

RodarFeel.eval(~S(@"2024-03-20".year), %{})
# => {:ok, 2024}

# Timezone-aware datetime
RodarFeel.eval(~S(date and time(@"2024-03-20T10:30:00", "America/New_York")), %{})
# => {:ok, #DateTime<2024-03-20 10:30:00-04:00 EDT America/New_York>}

# Context literals
RodarFeel.eval("{a: 1, b: a + 1}", %{})
# => {:ok, %{"a" => 1, "b" => 2}}

# For-in-return
RodarFeel.eval("for x in [1, 2, 3] return x * 2", %{})
# => {:ok, [2, 4, 6]}

# Quantified expressions
RodarFeel.eval("some x in [1, 2, 3] satisfies x > 2", %{})
# => {:ok, true}

# User-defined functions (lambdas)
RodarFeel.eval("{sq: function(x) x * x, result: sq(5)}", %{})
# => {:ok, %{"sq" => {:feel_function, ...}, "result" => 25}}

# DMN unary tests
RodarFeel.eval_unary("< 100", 50)
# => {:ok, true}

RodarFeel.eval_unary("[1..5]", 3)
# => {:ok, true}

RodarFeel.eval_unary("1, 2, 3", 2)
# => {:ok, true}

Features

Expression Language

• Arithmetic — +, -, *, /, %, **
• Comparison — =, !=, <, >, <=, >=
• Boolean logic — and, or, not (three-valued)
• Null propagation — nil + 1 evaluates to nil
• String concatenation — "hello" + " world" via +
• Path access — order.customer.name resolves nested maps
• Bracket access — data["key"] and items[0]
• If-then-else — if condition then a else b
• In operator — list membership and range checks
• Between — x between 1 and 10
• List literals — [1, 2, 3]
• Context literals — {a: 1, b: a + 1} with sequential evaluation
• For-in-return — for x in list return expr with cartesian product support
• Quantified — some/every x in list satisfies condition
• Instance of — x instance of number (all FEEL types)
• User-defined functions — function(x, y) x + y with closures
• Comments — // single-line, /* */ multi-line

Temporal Types

• Literals — @"2024-03-20" (date), @"10:30:00" (time), @"2024-03-20T10:30:00" (datetime)
• Timezone-aware — @"2024-03-20T10:30:00Z", @"2024-03-20T10:30:00+05:00", named zones via date and time(ndt, "America/New_York")
• Durations — @"P1Y2M" (year-month), @"PT1H30M" (day-time)
• Property access — .year, .month, .day, .hour, .minute, .second, .timezone, .offset
• Arithmetic — date + duration, date - date, datetime ± duration, time ± duration
• Comparison — dates, times, datetimes, and durations are comparable

DMN Unary Tests

Separate entry point for DMN decision table cells via RodarFeel.eval_unary/3:

• - — wildcard (match anything)
• < 100, >= 5, = "foo" — comparison tests
• [1..5], (1..5), [1..5), (1..5] — range tests (inclusive/exclusive/half-open)
• not(< 100) — negated test
• 1, 2, 3 — disjunction (match any)

Built-in Functions (48)

Architecture

• RodarFeel — public API (eval/2, eval_unary/3)
• RodarFeel.Parser — NimbleParsec-based parser producing AST tuples
• RodarFeel.Evaluator — tree-walking evaluator with null propagation and three-valued logic
• RodarFeel.Functions — built-in function dispatch (48 functions)
• RodarFeel.Duration — ISO 8601 duration struct with arithmetic and comparison

Dependencies

• nimble_parsec — parser combinators
• tz — IANA timezone database (lightweight, no hackney)

License

Apache-2.0. See LICENSE.md.