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Install L4 and write your first rule

Set up L4, write a short eligibility rule, and run it.

Install L4

VS Code extension

This is the main way to work with L4.

  1. Install VS Code.
  2. Install the L4 extension from the Marketplace.
  3. Open or create a file ending in .l4. The extension checks it as you type, shows errors and results, draws decision diagrams, and adds the L4 sidebar (Legalese AI, Render, Deploy).

The extension also ships the l4 command-line tool. The first time it starts, it asks "Install the L4 CLI?". Accept, or run L4: Install L4 CLI from the Command Palette later. On macOS and Linux it links l4 into ~/.local/bin; on Windows it copies l4.exe to %LOCALAPPDATA%\Programs\l4. Both are added to your PATH. Open a new terminal and check:

l4 --help

Web editor

To try L4 without installing anything, open the web editor at jl4.legalese.com.

Developers can also build l4 from source; see Using the l4 CLI.

Write the rule

The rule: a person is eligible if they are at least 18 years old and have valid ID.

Create a file called eligibility.l4 with this content:

-- eligibility-example.l4
-- A person is eligible if they are at least 18 years old and have valid ID.

§ `Eligibility`

DECLARE Person
    HAS `name`         IS A STRING
        `age`          IS A NUMBER
        `has valid ID` IS A BOOLEAN

GIVEN `the applicant` IS A Person
GIVETH A BOOLEAN
DECIDE `is eligible` IF
        `the applicant`'s `age` AT LEAST 18
    AND `the applicant`'s `has valid ID`

§§ `Examples`

`Alice` MEANS Person WITH
    `name`         IS "Alice"
    `age`          IS 25
    `has valid ID` IS TRUE

`Bob` MEANS Person WITH
    `name`         IS "Bob"
    `age`          IS 16
    `has valid ID` IS TRUE

`Charlie` MEANS Person WITH
    `name`         IS "Charlie"
    `age`          IS 30
    `has valid ID` IS FALSE

#EVAL `is eligible` `Alice`
#EVAL `is eligible` `Bob`
#EVAL `is eligible` `Charlie`

What each part does:

Part Meaning
-- ... A comment. L4 ignores it.
§ `Eligibility` A section heading. §§ starts a subsection.
DECLARE Person HAS ... A record type with three fields: a name (text), an age (number) and whether the person has valid ID (TRUE or FALSE).
GIVEN `the applicant` IS A Person The rule's input.
GIVETH A BOOLEAN The rule's answer is TRUE or FALSE.
DECIDE `is eligible` IF ... The rule itself: the answer is TRUE when the conditions hold. `the applicant`'s `age` reads a field of the input.
`Alice` MEANS Person WITH ... Example data: a person to test the rule on.
#EVAL `is eligible` `Alice` Evaluate the rule for Alice and show the result.

Names in backticks, such as `is eligible`, can contain spaces, so rules read like the text they encode. Indentation matters: the lines under HAS and IF are indented to show they belong there.

Run it

In VS Code or the web editor, results appear as you type. Hover over an #EVAL line to see its result; errors are underlined in red with an explanation.

On the command line:

l4 run eligibility.l4

Evaluation[1] @ eligibility.l4:34:1-28

Result:
  TRUE
...

There is one Evaluation block per #EVAL, in file order:

Expression Result Why
`is eligible` `Alice` TRUE 25 is at least 18; has ID
`is eligible` `Bob` FALSE 16 is under 18
`is eligible` `Charlie` FALSE No valid ID

l4 check eligibility.l4 only type-checks the file, without evaluating anything. It prints Check succeeded. when the file is valid.

Try a change: add a field `is banned` IS A BOOLEAN to Person, add AND NOT `the applicant`'s `is banned` to the rule, and give each example person a value for it.

Common mistakes

Missing indentation. Fields and conditions must be indented under the line that introduces them.

-- Wrong: the fields start in column 1
-- DECLARE Person
-- HAS `name` IS A STRING

-- Right
DECLARE Person
    HAS `name` IS A STRING

Missing 's in field access. Write `the applicant`'s `age`, not `the applicant` `age`.

Missing backticks around a name with spaces. Write DECIDE `is eligible` IF. Without backticks, is eligible is two separate words.

Next steps