Tlaplus Split Action

Split TLA+ Action into Two Actions

Split an existing TLA+ action into two sequential actions by introducing a new intermediate pc state. This is useful for modeling finer-grained atomicity or adding intermediate steps to an action sequence.

Prerequisites

• The specification must have a pc variable (or equivalent like state, phase) that tracks action sequencing. If not, create one.
• The action to split must update pc to transition between states

Overview

Given an action like:

ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_Start]
    /\ y' = y + 2
    /\ UNCHANGED >

Splitting it creates:

ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_ActionX_2]   \* Leads to new intermediate state
    /\ y' = y + 2                                   \* Original logic (or subset per user)
    /\ UNCHANGED >

ActionX_2(thread) ==                                \* New action
    /\ pc[thread] = PC_ActionX_2
    /\ pc' = [pc EXCEPT ![thread] = PC_Start]       \* Original destination
    /\ UNCHANGED >                        \* All vars unchanged (or subset per user)

Workflow

Step 0: Add pc variable if not present

If the specification lacks a pc variable, first use the "tlaplus-add-variable" skill to add pc variable. If the actions are thread- or process-specific, pc variable must be a function of the thread or process identifier to a string constant. For example,

PC_Begin == "PC_Begin"
PC_ActionX_1 == "PC_ActionX_1"
...

Init ==
    /\ pc = [thread \in Threads |-> PC_Begin]
    /\ ...

Step 1: Analyze the Target Action

Read the specification and identify:

1. The action to split
2. Its current pc guard (entry state)
3. Its pc' assignment (exit state)
4. All variable updates in the action
5. The UNCHANGED statement
6. The naming convention used (numbered suffix like _0, _1, etc.)

Step 2: Determine the New PC State Name

Follow the specification's naming convention:

If actions are numbered (e.g., ActionX_0, ActionX_1):

• Insert the new PC state between the split action and its successor
• Renumber subsequent PC states and actions

Example: Splitting ActionX_1 which goes to PC_Start:

• New intermediate state: PC_ActionX_2
• Original ActionX_1 now leads to PC_ActionX_2
• New action ActionX_2 leads to PC_Start

If actions have descriptive names (e.g., PC_Fetch, PC_Execute):

• Create a descriptive name for the intermediate state
• Ask the user if the name is unclear

Step 3: Add New PC State Constant

Add the new PC state to the PCStates set (or equivalent):

\* Before:
PCStates == {
    PC_Ready,
    PC_ActionX_1,
    ...
}

\* After:
PCStates == {
    PC_Ready,
    PC_ActionX_1, PC_ActionX_2,  \* Added PC_ActionX_2
    ...
}

Step 4: Modify the Original Action

Update the original action to:

1. Change pc' to transition to the NEW intermediate state
2. Update UNCHANGED to include variables that will be modified in the second action

Without user instructions (empty intermediate action):

\* Before:
ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_Start]
    /\ y' = y + 2
    /\ UNCHANGED >

\* After:
ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_ActionX_2]
    /\ y' = y + 2
    /\ UNCHANGED >

ActionX_2(thread) == ... will be created in the next step ...

With user instructions (specific split):

If user specifies which variables to update in first vs second action, follow those instructions:

\* User says: "Update x in first action, y in second"

\* Before:
ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_Start]
    /\ x' = x + 1
    /\ y' = y + 2
    /\ UNCHANGED >

\* After:
ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_ActionX_2]
    /\ x' = x + 1 \* User-specified update in first action
    /\ UNCHANGED >

ActionX_2(thread) == ... will be created in the next step ...

Step 5: Create the New Action:

Create the new action that:

1. Guards on the new intermediate PC state
2. Transitions to the ORIGINAL destination state
3. Contains the logic from the original action (or as specified by user)
4. Has appropriate UNCHANGED statement

ActionX_2(thread) ==                            \* New action
    /\ pc[thread] = PC_ActionX_2
    /\ pc' = [pc EXCEPT ![thread] = PC_Start]   \* Original destination
    /\ y' = y + 2                               \* Logic from original (or per user)
    /\ UNCHANGED >

Step 6: Renumber Subsequent Actions (if needed)

If the naming convention uses numbers and there are subsequent actions that would conflict:

Before splitting:

ActionX_0(thread) == ...  \* PC_ActionX_0 -> PC_ActionX_1
ActionX_1(thread) == ...  \* PC_ActionX_1 -> PC_Start (TO BE SPLIT)

After splitting:

ActionX_0(thread) == ...  \* PC_ActionX_0 -> PC_ActionX_1 (unchanged)
ActionX_1(thread) == ...  \* PC_ActionX_1 -> PC_ActionX_2 (new destination)
ActionX_2(thread) == ...  \* PC_ActionX_2 -> PC_Start (NEW ACTION)

If splitting ActionX_0 where ActionX_1 already exists, renumber:

Before:

ActionX_0(thread) == ...  \* PC_ActionX_0 -> PC_ActionX_1 (TO BE SPLIT)
ActionX_1(thread) == ...  \* PC_ActionX_1 -> PC_Start

After:

ActionX_0(thread) == ...  \* PC_ActionX_0 -> PC_ActionX_1 (NEW intermediate)
ActionX_1(thread) == ...  \* PC_ActionX_1 -> PC_ActionX_2 (renumbered from old ActionX_1)
ActionX_2(thread) == ...  \* PC_ActionX_2 -> PC_Start (renumbered)

Step 7: Update TypeOk Predicate

If TypeOk contains PCStates enumeration, add the new PC state:

\* Before:
TypeOk ==
    /\ pc \in [Threads -> PCStates]
    ...

\* If PCStates is defined separately, it was updated in Step 3.
\* If PCStates is inline, update it here.

Step 8: Update Next Predicate (if needed)

If the Next predicate explicitly lists actions (not common), add the new action:

Next == \E thread \in Threads :
    \/ ActionX_0(thread)
    \/ ActionX_1(thread)
    \/ ActionX_2(thread)   \* Add new action
    ...

If actions are grouped (e.g., ActionX(thread) with internal disjunctions), the new disjunct is already included.

Step 9: Update Fairness Constraints (if needed)

If fairness constraints reference specific PC states or actions, update them:

\* Before:
Fairness ==
    /\ WF_vars(ActionX_1(thread))

\* After - add fairness for new action:
Fairness ==
    /\ WF_vars(ActionX_1(thread))
    /\ WF_vars(ActionX_2(thread))

UNCHANGED Statement Rules

When splitting an action, follow these rules for UNCHANGED:

1. First action (intermediate step):

• If user specifies no variable updates: add ALL non-pc variables to UNCHANGED
• If user specifies some updates: add remaining variables to UNCHANGED

1. Second action (new action):

• Contains variables that are NOT modified in the new action
• Typically mirrors the original UNCHANGED (unless user specifies otherwise)

1. Never forget variables: Every variable must either be primed (updated) or in UNCHANGED

Verification Checklist

After splitting, verify:

• [ ] New PC state constant is defined
• [ ] New PC state is in PCStates set
• [ ] Original action transitions to new intermediate PC state
• [ ] New action guards on new PC state
• [ ] New action transitions to original destination
• [ ] All variables are either primed or in UNCHANGED (both actions)
• [ ] Naming follows spec convention (renumbered if needed)
• [ ] TypeOk updated if needed
• [ ] Next predicate includes new action (if explicitly listed)
• [ ] Fairness constraints updated if present

Example: Full Split

User request: "Split ActionX_1 into two actions"

Before:

PC_ActionX_1 == "PC_ActionX_1"

PCStates == { PC_Ready, PC_ActionX_1 }

ActionX_0(thread) ==
    /\ pc[thread] = PC_Start
    /\ pc' = [pc EXCEPT ![thread] = PC_ActionX_1]
    /\ x' = x + 1
    /\ UNCHANGED >

ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_Start]
    /\ y' = y + 2
    /\ UNCHANGED >

After:

PC_ActionX_1 == "PC_ActionX_1"
PC_ActionX_2 == "PC_ActionX_2"  \* NEW

PCStates == { PC_Ready, PC_ActionX_1, PC_ActionX_2 }  \* Updated

ActionX_0(thread) ==
    /\ pc[thread] = PC_Start
    /\ pc' = [pc EXCEPT ![thread] = PC_ActionX_1]
    /\ x' = x + 1
    /\ UNCHANGED >

ActionX_1(thread) ==
    /\ pc[thread] = PC_ActionX_1
    /\ pc' = [pc EXCEPT ![thread] = PC_ActionX_2]  \* Changed destination
    /\ UNCHANGED >                        \* All vars unchanged

ActionX_2(thread) ==                                \* NEW ACTION
    /\ pc[thread] = PC_ActionX_2
    /\ pc' = [pc EXCEPT ![thread] = PC_Start]
    /\ y' = y + 2
    /\ UNCHANGED >

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: tlaplus
• Source: tlaplus/AgentSkills
• License: MIT

Install and usage instructions live in the source repository linked above.