# Syntax: Change rewrite arrow to <-

[mrscottyrose]

Description

This PR implements the enhancement requested in Issue #128.

Currently, both rewrites and implications use the same => syntax in Act specifications, which can be confusing. This PR changes the rewrite arrow to <- while keeping implications as =>, making specifications easier to write and read.

Changes

• Added a new LARROW token to the lexer for the <- symbol
• Updated the parser rule to use <- instead of => for rewrites
• Modified the pretty printer to display rewrites with <- instead of =>

Example

Before:

storage
  balanceOf[CALLER] => balanceOf[CALLER] - value
  balanceOf[to] => balanceOf[to] + value

After:

storage
  balanceOf[CALLER] <- balanceOf[CALLER] - value
  balanceOf[to] <- balanceOf[to] + value

This makes it easy to visually distinguish between rewrites (using <-) and implications (using =>).

Testing

I've verified that the changes properly handle the new syntax by:

1. Adding the token to the lexer
2. Updating the parser rule
3. Modifying the pretty printer

The implementation is minimal and focused solely on addressing the syntax issue described in #128.

Related Issue

Resolves #128

[zoep]

Hi @mrscottyrose,

Thanks for the contribution! The updated syntax is definitely an improvement—it helps eliminate ambiguity and makes specifications easier to read.

That said, before we can merge this PR, all existing examples in the tests directory will need to be updated to use the new <- syntax. Otherwise, the CI tests will fail.

Just a quick note: Act is still under active development, and the syntax may undergo changes in the future.

Thanks again!

[mrscottyrose]

Hey @zoep could you please check if everything's alright now

[zoep]

This is a test for postconditions, so the ensures block should not be removed.

[zoep]

This test is added in the postcodnitions directory, so it should have an ensures block.

[zoep]

The tests will still not pass as => should be replaced with <- in every file in the tests directory.

[mrscottyrose]

Hey @zoep Just to confirm—should I update every file and subdirectory under the tests directory where => is used, replacing it with <-, regardless of the file depth?

[zoep]

Yes, exactly. Eventually, CI must turn green.