feat: chain-step evidence composition — expand evidence steps in chains

A chain step that references a derived evidence is now expanded at compile
time, keeping the engine a flat edge-traversal evaluator:

- DIRECT evidence step -> renamed to its underlying relation
  (member_of(user,*g){ group_read(g,doc) } where group_read = can_view
  becomes step 'can_view').
- CHAIN evidence step -> its steps are spliced into the parent chain
  (a sub-path flattens into the linear source->...->object traversal).
- Any other evidence type (defeasible/logical/comparator) as a step is a
  compile-time error: it is a condition, not an edge traversal.
- Cycles and self-references through chain steps are compile-time errors
  (the existing composition cycle guard now covers steps).

Rigor: oracle campaign gains a chain_step_composition construct; illegal
mutations gain a non-lowerable-chain-step case. Fixture suites updated to
retarget the self-recursive 'canRead/canAccess/...' terminals (an unsupported
recursion pattern that now fails loudly) to an any-typed 'reachable' fact,
preserving the nested-pattern parsing intent.

tests/ChainStepComposition.test.js

@@ -0,0 +1,123 @@
+/**
+ * tests/ChainStepComposition.test.js — evidence composition inside CHAIN
+ * steps. A chain step that references a derived evidence is expanded at
+ * compile time:
+ *   - a DIRECT evidence step → renamed to its underlying relation
+ *     (member_of(user,*g){ group_read(g,doc) } where group_read = can_view
+ *      becomes step 'can_view');
+ *   - a CHAIN evidence step → its steps are spliced into the parent chain
+ *     (a sub-path flattens into the linear source→…→object traversal);
+ *   - a DEFEASIBLE / LOGICAL / COMPARATOR evidence step is not an edge
+ *     traversal and is rejected at compile time;
+ *   - cycles and self-references through chain steps are compile errors.
+ */
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { Arbiter } from '@arbiter/core';
+import { DSLCompiler } from '../src/DSLCompiler.js';
+
+const DEFS = `
+  definition Employee { id: string }
+  definition Group { id: string }
+  definition Doc { id: string }
+  fact member_of(user: Employee, group: Group)
+  fact group_has(group: Group, sub: Group)
+  fact can_view(group: Group, doc: Doc)
+  fact can_access(group: Group, doc: Doc)
+  fact banned(group: Group)
+`;
+
+function compile(dsl, name = 'chain-compose') {
+  const arb = new Arbiter();
+  const compiler = new DSLCompiler(arb);
+  const result = compiler.compile(dsl, name);
+  return { arb, result };
+}
+
+describe('Chain step composition', () => {
+  it('renames a direct-evidence chain step to its underlying relation', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence group_read(group: Group, doc: Doc) { can_view(group, doc) }
+      evidence can_via(user: Employee, doc: Doc) { member_of(user, *g) { group_read(g, doc) } }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    // step 'group_read' → 'can_view'
+    assert.deepEqual(arb.relationConfigs.get('can_via').steps, ['member_of', 'can_view']);
+    arb.addNode('u:1', 'Employee'); arb.addNode('g:1', 'Group'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'member_of', 'g:1', { possibility: 1.0 });
+    arb.addRelation('g:1', 'can_view', 'doc:9', { possibility: 0.7 });
+    assert.equal(arb.check('u:1', 'can_via', 'doc:9').possibility, 0.7);
+  });
+
+  it('splices a chain-evidence step into the parent chain', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence group_enter(group: Group, doc: Doc) { group_has(group, *s) { can_access(s, doc) } }
+      evidence can_deep(user: Employee, doc: Doc) { member_of(user, *g) { group_enter(g, doc) } }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    // step 'group_enter' → its steps [group_has, can_access]
+    assert.deepEqual(arb.relationConfigs.get('can_deep').steps, ['member_of', 'group_has', 'can_access']);
+    arb.addNode('u:1', 'Employee'); arb.addNode('g:1', 'Group'); arb.addNode('g2:2', 'Group'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'member_of', 'g:1', { possibility: 1.0 });
+    arb.addRelation('g:1', 'group_has', 'g2:2', { possibility: 0.9 });
+    arb.addRelation('g2:2', 'can_access', 'doc:9', { possibility: 0.8 });
+    assert.equal(arb.check('u:1', 'can_deep', 'doc:9').possibility, 0.8);
+  });
+
+  it('expands a chain step whose direct evidence is itself composed', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence group_view(group: Group, doc: Doc) { can_view(group, doc) }
+      evidence group_read(group: Group, doc: Doc) { group_view(group, doc) }
+      evidence can_via(user: Employee, doc: Doc) { member_of(user, *g) { group_read(g, doc) } }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    assert.deepEqual(arb.relationConfigs.get('can_via').steps, ['member_of', 'can_view']);
+    arb.addNode('u:1', 'Employee'); arb.addNode('g:1', 'Group'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'member_of', 'g:1', { possibility: 1.0 });
+    arb.addRelation('g:1', 'can_view', 'doc:9', { possibility: 0.6 });
+    assert.equal(arb.check('u:1', 'can_via', 'doc:9').possibility, 0.6);
+  });
+
+  it('rejects a defeasible/logical evidence as a chain step', () => {
+    const { result } = compile(`
+      ${DEFS}
+      evidence gated(group: Group, doc: Doc) { WHEN can_view(group, doc) UNLESS banned(group) }
+      evidence can_via(user: Employee, doc: Doc) { member_of(user, *g) { gated(g, doc) } }
+    `);
+    assert.equal(result.success, false);
+    assert.ok(result.errors.some(e => /Chain step 'gated'/.test(e)), JSON.stringify(result.errors));
+  });
+
+  it('rejects a mutual cycle through chain steps', () => {
+    const { result } = compile(`
+      ${DEFS}
+      evidence cyc_a(group: Group, doc: Doc) { group_has(group, *g) { cyc_b(g, doc) } }
+      evidence cyc_b(group: Group, doc: Doc) { cyc_a(group, doc) }
+    `);
+    assert.equal(result.success, false);
+    assert.ok(result.errors.some(e => /[Cc]yclic/.test(e)), JSON.stringify(result.errors));
+  });
+
+  it('rejects a self-reference through its own chain step', () => {
+    const { result } = compile(`
+      ${DEFS}
+      evidence cyc_c(group: Group, doc: Doc) { group_has(group, *g) { cyc_c(g, doc) } }
+    `);
+    assert.equal(result.success, false);
+    assert.ok(result.errors.some(e => /[Cc]yclic/.test(e)), JSON.stringify(result.errors));
+  });
+
+  it('re-derives transitive dependencies through expanded chain steps', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence group_read(group: Group, doc: Doc) { can_view(group, doc) }
+      evidence can_via(user: Employee, doc: Doc) { member_of(user, *g) { group_read(g, doc) } }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    // dependsOn reflects the expanded step, not the evidence reference
+    assert.deepEqual(arb.relationConfigs.get('can_via').dependsOn, ['member_of', 'can_view']);
+  });
+});

tests/EvidenceTests.js

@@ -45,6 +45,7 @@ const DSL_SUPPORT = `
   fact isMember(user: any, group: any)
   fact isFriend(user: any, friend: any)
   fact similar(a: any, b: any)
+  fact reachable(user: any, doc: any)
   fact parentOf(user: any, parent: any)
   fact isEditable(doc: any)
   fact isPublic(doc: any)
@@ -159,7 +160,7 @@ describe('Evidence Rules', () => {
       {
         input: `evidence canRead(user: Employee, doc: Document) {
           isMember(user, *group) {
-            canRead(group, doc)
+            reachable(group, doc)
           }
         }`,
         description: 'Basic pattern matching with wildcard'
@@ -167,7 +168,7 @@ describe('Evidence Rules', () => {
       {
         input: `evidence canRead(user: Employee, doc: Document) {
           isMember(user, *group) {
-            canRead(group, doc)
+            reachable(group, doc)
           } limit 5
         }`,
         description: 'Pattern matching with limit'
@@ -175,7 +176,7 @@ describe('Evidence Rules', () => {
       {
         input: `evidence canRead(user: Employee, doc: Document) {
           similar(doc, *similar) |similarity| {
-            canRead(user, similar)
+            reachable(user, similar)
           } with similarity > 0.7
         }`,
         description: 'Pattern matching with binding and condition'
@@ -183,7 +184,7 @@ describe('Evidence Rules', () => {
       {
         input: `evidence canRead(user: Employee, doc: Document) {
           similar(doc, *similar) |similarity| {
-            canRead(user, similar)
+            reachable(user, similar)
           } limit 5 with similarity > 0.7
         }`,
         description: 'Pattern matching with binding, condition, and limit'
@@ -192,7 +193,7 @@ describe('Evidence Rules', () => {
         input: `evidence canRead(user: Employee, doc: Document) {
           isMember(user, *group) {
             isMember(group, *parentGroup) {
-              canRead(parentGroup, doc)
+              reachable(parentGroup, doc)
             } limit 2
           } limit 3
         }`,
@@ -202,7 +203,7 @@ describe('Evidence Rules', () => {
         input: `evidence canRead(user: Employee, doc: Document) {
           isFriend(user, *friend) {
             isMember(friend, *group) {
-              canRead(group, doc)
+              reachable(group, doc)
             } limit 1
           } limit 5
         }`,
@@ -291,15 +292,15 @@ describe('Evidence Rules', () => {
           owns(user, doc)
 
           isMember(user, *group) {
-            canRead(group, doc)
+            reachable(group, doc)
           } limit 5
 
           parentOf(user, *parent) {
-            canRead(parent, doc)
+            reachable(parent, doc)
           } limit 3
 
           similar(doc, *similar) |similarity| {
-            canRead(user, similar)
+            reachable(user, similar)
           } limit 5 with similarity > 0.7
 
           WHEN hasRole(user, 'admin') UNLESS isSuspended(user)
@@ -337,11 +338,11 @@ describe('Evidence Rules', () => {
           owns(user, doc)
 
           isMember(user, *group) {
-            canModify(group, doc)
+            reachable(group, doc)
           } limit 3
 
           similar(doc, *similar) |similarity| {
-            canModify(user, similar)
+            reachable(user, similar)
             isEditable(similar)
           } limit 2 with similarity > 0.8
 
@@ -385,7 +386,7 @@ describe('Evidence Rules', () => {
       {
         input: `evidence canRead(user: Employee, doc: Document) {
           isMember(user, *group) {
-            canRead(group, doc)
+            reachable(group, doc)
           } with
         }`,
         description: 'Incomplete with clause should fail'
@@ -393,7 +394,7 @@ describe('Evidence Rules', () => {
       {
         input: `evidence canRead(user: Employee, doc: Document) {
           isMember(user, *group) {
-            canRead(group, doc)
+            reachable(group, doc)
           } limit
         }`,
         description: 'Incomplete limit should fail'

tests/IntegrationTests.js

@@ -105,6 +105,7 @@ describe('Integration Tests', () => {
       fact hasAccess(user: Employee, resource: Resource, level: string) CACHE lazy
       fact isColleague(user: any, colleague: any) symmetrical CACHE lazy limit 50
       fact isParentOf(parent: Employee, child: Employee) transitive CACHE eager limit 3
+      fact reachable(user: any, doc: any) CACHE lazy
       fact hasClearance(user: Employee, level: string) CACHE eager
       fact parentOf(user: any, parent: any) CACHE eager
       fact similar(a: any, b: any) CACHE lazy
@@ -119,15 +120,15 @@ describe('Integration Tests', () => {
         owns(user, doc)
 
         isMember(user, *group) {
-          canRead(group, doc)
+          reachable(group, doc)
         } limit 5
 
         parentOf(user, *parent) {
-          canRead(parent, doc)
+          reachable(parent, doc)
         } limit 3
 
         similar(doc, *similar) |similarity| {
-          canRead(user, similar)
+          reachable(user, similar)
         } limit 5 with similarity > 0.7
 
         WHEN hasRole(user, 'admin') UNLESS isSuspended(user)
@@ -137,7 +138,7 @@ describe('Integration Tests', () => {
         owns(user, doc)
 
         isMember(user, *group) {
-          canWrite(group, doc)
+          reachable(group, doc)
         } limit 3
 
         WHEN hasRole(user, 'admin') UNLESS isSuspended(user)
@@ -330,6 +331,7 @@ describe('Integration Tests', () => {
 
       fact isMember(user: any, org: any) transitive CACHE lazy limit 5
       fact isParentOf(parent: Organization, child: Organization) transitive CACHE eager limit 3
+      fact reachable(user: any, doc: any) CACHE lazy
       fact hasRole(user: Employee, role: string) CACHE eager
       fact hasClearance(user: Employee, level: string) CACHE eager
       fact isSuspended(user: any) CACHE lazy
@@ -339,7 +341,7 @@ describe('Integration Tests', () => {
         isMember(user, org)
 
         isParentOf(org, *parentOrg) {
-          canAccessOrg(user, parentOrg)
+          reachable(user, parentOrg)
         } limit 3
 
         WHEN hasRole(user, 'admin') UNLESS isSuspended(user)
@@ -347,11 +349,11 @@ describe('Integration Tests', () => {
 
       evidence canAccessResource(user: Employee, resource: Resource) {
         isMember(user, *org) {
-          canAccessResource(org, resource)
+          reachable(org, resource)
         } limit 5
 
         parentOf(user, *parent) {
-          canAccessResource(parent, resource)
+          reachable(parent, resource)
         } limit 2
       }
     `;
@@ -379,6 +381,7 @@ describe('Integration Tests', () => {
       fact hasInterest(user: any, interest: string) CACHE lazy
       fact hasTag(doc: any, tag: string) CACHE lazy
       fact owns(user: any, doc: any) CACHE eager
+      fact reachable(user: any, doc: any) CACHE lazy
       fact similar(a: any, b: any) CACHE lazy
       fact isPublic(doc: any) CACHE eager
       fact hasInterests(user: any) CACHE lazy
@@ -390,12 +393,12 @@ describe('Integration Tests', () => {
         owns(user, doc)
 
         similar(doc, *similar) |similarity| {
-          canRead(user, similar)
+          reachable(user, similar)
           isPublic(similar)
         } limit 10 with similarity > 0.7
 
         isFriend(user, *friend) {
-          canRead(friend, doc)
+          reachable(friend, doc)
         } limit 5
 
         fusion majority {
@@ -406,7 +409,7 @@ describe('Integration Tests', () => {
 
       evidence canRecommend(user: Employee, doc: Document) {
         similar(user, *similarUser) |similarity| {
-          canRead(similarUser, doc)
+          reachable(similarUser, doc)
         } limit 20 with similarity > 0.8
 
         fusion average {
@@ -556,13 +559,14 @@ describe('Integration Tests', () => {
       fact isFriend(user: any, friend: any) symmetrical CACHE eager limit 50
       fact hasPermission(user: Employee, resource: Resource, action: string) CACHE eager
       fact owns(user: Employee, resource: Resource) CACHE eager
+  fact reachable(user: any, doc: any) CACHE lazy
 
       // Optimized evidence rules
       evidence canAccess(user: Employee, resource: Resource) {
         owns(user, resource)
 
         isMember(user, *group) {
-          canAccess(group, resource)
+          reachable(group, resource)
         } limit 3
 
         WHEN hasPermission(user, resource, 'read')
@@ -572,7 +576,7 @@ describe('Integration Tests', () => {
         owns(user, resource)
 
         isMember(user, *group) {
-          canModify(group, resource)
+          reachable(group, resource)
         } limit 2
 
         WHEN hasPermission(user, resource, 'write')

tests/rigor/dsl-generative-oracle.test.js

@@ -32,6 +32,7 @@ const FACTS = `
   fact can_access(group: Group, doc: Doc)
   fact owner(group: Group, doc: Doc)
   fact granted(user: Employee, doc: Doc)
+  fact group_perm(group: Group, doc: Doc)
   fact banned(user: Employee)
   fact mfa(user: Employee)
 `;
@@ -116,6 +117,17 @@ function buildProgram(kind, ps) {
       oracle = pOwn;
       break;
     }
+    case 'chain_step_composition': {
+      // group_read (a direct evidence) used as a CHAIN STEP inside can_via:
+      // the step is expanded at compile time to the underlying can_view edge.
+      const [pm, pv] = ps;
+      evidence = `evidence group_read(group: Group, doc: Doc) { group_perm(group, doc) }
+                  evidence can_via(user: Employee, doc: Doc) { member_of(user, *g) { group_read(g, doc) } }`;
+      edges.push({ src: 'u:1', relation: 'member_of', dst: 'g:1', possibility: pm });
+      edges.push({ src: 'g:1', relation: 'group_perm', dst: 'doc:9', possibility: pv });
+      oracle = Math.min(pm, pv);
+      break;
+    }
     default:
       throw new Error(`unknown construct: ${kind}`);
   }
@@ -151,7 +163,7 @@ function runCheck({ kind, ps }) {
 }
 
 const CONSTRUCTS = ['direct', 'chain', 'tuple_to_userset', 'fusion_min', 'fusion_max',
-  'when_unless', 'never_always', 'requires_when', 'composition'];
+  'when_unless', 'never_always', 'requires_when', 'composition', 'chain_step_composition'];
 
 describe('DSL generative oracle parity (rigor)', () => {
   it('generated legal DSL compiles and every check matches the oracle', async () => {

tests/rigor/dsl-illegal-mutations.test.js

@@ -96,6 +96,14 @@ const MUTATIONS = {
       'evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }',
       'fact can_read(user: Employee, doc: Doc)\n  evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }'
     )
+  },
+  non_lowerable_chain_step: {
+    desc: 'a defeasible evidence used as a chain step (cannot lower to an edge)',
+    mustFail: true,
+    apply: () => VALID_DSL.replace(
+      'evidence can_enter(user: Employee, doc: Doc) { member_of(user, *g) { can_access(g, doc) } }',
+      'evidence can_gated(group: Group, doc: Doc) { WHEN can_access(group, doc) UNLESS banned(group) }\n  evidence can_enter(user: Employee, doc: Doc) { member_of(user, *g) { can_gated(g, doc) } }'
+    ).replace('fact can_access(group: Group, doc: Doc)', 'fact can_access(group: Group, doc: Doc)\n  fact banned(group: Group)')
   }
 };