diff --git a/lib/compiler/index.js b/lib/compiler/index.js
index c0515dc..16bd832 100644
--- a/lib/compiler/index.js
+++ b/lib/compiler/index.js
@@ -10,6 +10,7 @@ const reportInfiniteRecursion = require( "./passes/report-infinite-recursion" );
 const reportInfiniteRepetition = require( "./passes/report-infinite-repetition" );
 const reportUndefinedRules = require( "./passes/report-undefined-rules" );
 const inferenceMatchResult = require( "./passes/inference-match-result" );
+const Session = require( "./session" );
 const util = require( "../util" );
 
 function processOptions( options, defaults ) {
@@ -25,6 +26,8 @@ function processOptions( options, defaults ) {
 
 const compiler = {
 
+    Session: Session,
+
     // Compiler passes.
     //
     // Each pass is a function that is passed the AST. It can perform checks on it
@@ -53,7 +56,7 @@ const compiler = {
     // if the AST contains a semantic error. Note that not all errors are detected
     // during the generation and some may protrude to the generated parser and
     // cause its malfunction.
-    compile( ast, passes, options ) {
+    compile( ast, session, options ) {
 
         options = typeof options !== "undefined" ? options : {};
 
@@ -68,11 +71,11 @@ const compiler = {
             trace: false
         } );
 
-        util.each( passes, stage => {
+        util.each( session.passes, stage => {
 
             stage.forEach( pass => {
 
-                pass( ast, options );
+                pass( ast, session, options );
 
             } );
 
diff --git a/lib/compiler/passes/calc-report-failures.js b/lib/compiler/passes/calc-report-failures.js
index 70d3676..23b8c4b 100644
--- a/lib/compiler/passes/calc-report-failures.js
+++ b/lib/compiler/passes/calc-report-failures.js
@@ -1,11 +1,9 @@
 "use strict";
 
-const visitor = require( "../../ast" ).visitor;
-
 // Determines if rule always used in disabled report failure context,
 // that means, that any failures, reported within it, are never will be
 // visible, so the no need to report it.
-function calcReportFailures( ast, options ) {
+function calcReportFailures( ast, session, options ) {
 
     // By default, not report failures for rules...
     ast.rules.forEach( rule => {
@@ -26,7 +24,7 @@ function calcReportFailures( ast, options ) {
 
     } );
 
-    const calc = visitor.build( {
+    const calc = session.buildVisitor( {
         rule( node ) {
 
             calc( node.expression );
diff --git a/lib/compiler/passes/generate-bytecode.js b/lib/compiler/passes/generate-bytecode.js
index 2e679fe..4b89a05 100644
--- a/lib/compiler/passes/generate-bytecode.js
+++ b/lib/compiler/passes/generate-bytecode.js
@@ -1,7 +1,5 @@
 "use strict";
 
-const op = require( "../opcodes" );
-const visitor = require( "../../ast" ).visitor;
 const util = require( "../../util" );
 
 // Generates bytecode.
@@ -199,7 +197,9 @@ const util = require( "../../util" );
 //          }
 //          expected.top().variants.pushAll(value.variants);
 //        }
-function generateBytecode( ast ) {
+function generateBytecode( ast, session ) {
+
+    const op = session.opcodes;
 
     const literals = [];
     const classes = [];
@@ -333,7 +333,7 @@ function generateBytecode( ast ) {
 
     }
 
-    generate = visitor.build( {
+    generate = session.buildVisitor( {
         grammar( node ) {
 
             node.rules.forEach( generate );
diff --git a/lib/compiler/passes/generate-js.js b/lib/compiler/passes/generate-js.js
index 2d29776..fcb4cc0 100644
--- a/lib/compiler/passes/generate-js.js
+++ b/lib/compiler/passes/generate-js.js
@@ -3,10 +3,11 @@
 "use strict";
 
 const js = require( "../js" );
-const op = require( "../opcodes" );
 
 // Generates parser JavaScript code.
-function generateJS( ast, options ) {
+function generateJS( ast, session, options ) {
+
+    const op = session.opcodes;
 
     /* These only indent non-empty lines to avoid trailing whitespace. */
     const lineMatchRE = /^([^`\r\n]+?(?:`[^`]*?`[^\r\n]*?)?)$/gm;
diff --git a/lib/compiler/passes/inference-match-result.js b/lib/compiler/passes/inference-match-result.js
index 8aed1d4..2546c4d 100644
--- a/lib/compiler/passes/inference-match-result.js
+++ b/lib/compiler/passes/inference-match-result.js
@@ -1,13 +1,12 @@
 "use strict";
 
-const visitor = require( "../../ast" ).visitor;
 const GrammarError = require( "../../grammar-error" );
 
 // Inference match result of the rule. Can be:
 //   -1: negative result, always fails
 //    0: neutral result, may be fail, may be match
 //    1: positive result, always match
-function inferenceMatchResult( ast ) {
+function inferenceMatchResult( ast, session ) {
 
     let inference;
     function sometimesMatch( node ) {
@@ -72,7 +71,7 @@ function inferenceMatchResult( ast ) {
 
     }
 
-    inference = visitor.build( {
+    inference = session.buildVisitor( {
         rule( node ) {
 
             let oldResult;
diff --git a/lib/compiler/passes/remove-proxy-rules.js b/lib/compiler/passes/remove-proxy-rules.js
index db32cc5..690dc08 100644
--- a/lib/compiler/passes/remove-proxy-rules.js
+++ b/lib/compiler/passes/remove-proxy-rules.js
@@ -1,9 +1,7 @@
 "use strict";
 
-const visitor = require( "../../ast" ).visitor;
-
 // Removes proxy rules -- that is, rules that only delegate to other rule.
-function removeProxyRules( ast, options ) {
+function removeProxyRules( ast, session, options ) {
 
     function isProxyRule( node ) {
 
@@ -13,7 +11,7 @@ function removeProxyRules( ast, options ) {
 
     function replaceRuleRefs( ast, from, to ) {
 
-        const replace = visitor.build( {
+        const replace = session.buildVisitor( {
             rule_ref( node ) {
 
                 if ( node.name === from ) {
diff --git a/lib/compiler/passes/report-duplicate-labels.js b/lib/compiler/passes/report-duplicate-labels.js
index 28564fc..5cf796b 100644
--- a/lib/compiler/passes/report-duplicate-labels.js
+++ b/lib/compiler/passes/report-duplicate-labels.js
@@ -1,12 +1,11 @@
 "use strict";
 
 const GrammarError = require( "../../grammar-error" );
-const visitor = require( "../../ast" ).visitor;
 const util = require( "../../util" );
 const __hasOwnProperty = Object.prototype.hasOwnProperty;
 
 // Checks that each label is defined only once within each scope.
-function reportDuplicateLabels( ast ) {
+function reportDuplicateLabels( ast, session ) {
 
     let check;
 
@@ -16,7 +15,7 @@ function reportDuplicateLabels( ast ) {
 
     }
 
-    check = visitor.build( {
+    check = session.buildVisitor( {
         rule( node ) {
 
             check( node.expression, {} );
diff --git a/lib/compiler/passes/report-duplicate-rules.js b/lib/compiler/passes/report-duplicate-rules.js
index 287554b..82da138 100644
--- a/lib/compiler/passes/report-duplicate-rules.js
+++ b/lib/compiler/passes/report-duplicate-rules.js
@@ -1,15 +1,14 @@
 "use strict";
 
 const GrammarError = require( "../../grammar-error" );
-const visitor = require( "../../ast" ).visitor;
 const __hasOwnProperty = Object.prototype.hasOwnProperty;
 
 // Checks that each rule is defined only once.
-function reportDuplicateRules( ast ) {
+function reportDuplicateRules( ast, session ) {
 
     const rules = {};
 
-    const check = visitor.build( {
+    const check = session.buildVisitor( {
         rule( node ) {
 
             const name = node.name;
diff --git a/lib/compiler/passes/report-infinite-recursion.js b/lib/compiler/passes/report-infinite-recursion.js
index def253f..1f6c270 100644
--- a/lib/compiler/passes/report-infinite-recursion.js
+++ b/lib/compiler/passes/report-infinite-recursion.js
@@ -1,7 +1,6 @@
 "use strict";
 
 const GrammarError = require( "../../grammar-error" );
-const visitor = require( "../../ast" ).visitor;
 
 // Reports left recursion in the grammar, which prevents infinite recursion in
 // the generated parser.
@@ -13,11 +12,11 @@ const visitor = require( "../../ast" ).visitor;
 //
 // In general, if a rule reference can be reached without consuming any input,
 // it can lead to left recursion.
-function reportInfiniteRecursion( ast ) {
+function reportInfiniteRecursion( ast, session ) {
 
     const visitedRules = [];
 
-    const check = visitor.build( {
+    const check = session.buildVisitor( {
         rule( node ) {
 
             visitedRules.push( node.name );
diff --git a/lib/compiler/passes/report-infinite-repetition.js b/lib/compiler/passes/report-infinite-repetition.js
index 72d9dc9..1125f53 100644
--- a/lib/compiler/passes/report-infinite-repetition.js
+++ b/lib/compiler/passes/report-infinite-repetition.js
@@ -1,13 +1,12 @@
 "use strict";
 
 const GrammarError = require( "../../grammar-error" );
-const visitor = require( "../../ast" ).visitor;
 
 // Reports expressions that don't consume any input inside |*| or |+| in the
 // grammar, which prevents infinite loops in the generated parser.
-function reportInfiniteRepetition( ast ) {
+function reportInfiniteRepetition( ast, session ) {
 
-    const check = visitor.build( {
+    const check = session.buildVisitor( {
         zero_or_more( node ) {
 
             if ( ! ast.alwaysConsumesOnSuccess( node.expression ) ) {
diff --git a/lib/compiler/passes/report-undefined-rules.js b/lib/compiler/passes/report-undefined-rules.js
index ea8ef24..1b47f46 100644
--- a/lib/compiler/passes/report-undefined-rules.js
+++ b/lib/compiler/passes/report-undefined-rules.js
@@ -1,12 +1,11 @@
 "use strict";
 
 const GrammarError = require( "../../grammar-error" );
-const visitor = require( "../../ast" ).visitor;
 
 // Checks that all referenced rules exist.
-function reportUndefinedRules( ast, options ) {
+function reportUndefinedRules( ast, session, options ) {
 
-    const check = visitor.build( {
+    const check = session.buildVisitor( {
         rule_ref( node ) {
 
             if ( ! ast.findRule( node.name ) ) {
diff --git a/lib/compiler/session.js b/lib/compiler/session.js
new file mode 100644
index 0000000..a33ac13
--- /dev/null
+++ b/lib/compiler/session.js
@@ -0,0 +1,35 @@
+"use strict";
+
+const ast = require( "../ast" );
+const opcodes = require( "./opcodes" );
+const parser = require( "../parser" );
+
+class Session {
+
+    constructor( options ) {
+
+        options = typeof options !== "undefined" ? options : {};
+
+        this.grammar = options.grammar;
+        this.opcodes = options.opcodes || opcodes;
+        this.parser = options.parser || parser;
+        this.passes = options.passes || [];
+        this.visitor = options.visitor || ast.visitor;
+
+    }
+
+    parse( input, options ) {
+
+        return this.parser.parse( input, options );
+
+    }
+
+    buildVisitor( functions ) {
+
+        return this.visitor.build( functions );
+
+    }
+
+}
+
+module.exports = Session;
diff --git a/lib/peg.js b/lib/peg.js
index 66b2d70..d742e6a 100644
--- a/lib/peg.js
+++ b/lib/peg.js
@@ -30,20 +30,20 @@ const peg = {
         options = typeof options !== "undefined" ? options : {};
 
         const plugins = "plugins" in options ? options.plugins : [];
-        const config = {
-            parser: parser,
+        const session = new compiler.Session( {
+            grammar: grammar,
             passes: util.convertPasses( compiler.passes )
-        };
+        } );
 
         plugins.forEach( p => {
 
-            p.use( config, options );
+            p.use( session, options );
 
         } );
 
         return compiler.compile(
-            config.parser.parse( grammar, options.parser || {} ),
-            config.passes,
+            session.parse( grammar, options.parser || {} ),
+            session,
             options
         );
 
diff --git a/test/spec/unit/compiler/passes/helpers.js b/test/spec/unit/compiler/passes/helpers.js
index 533db6b..1cd5469 100644
--- a/test/spec/unit/compiler/passes/helpers.js
+++ b/test/spec/unit/compiler/passes/helpers.js
@@ -1,7 +1,7 @@
 "use strict";
 
 const LikeHelper = require( "chai-like" );
-const parser = require( "pegjs-dev" ).parser;
+const Session = require( "pegjs-dev" ).compiler.Session;
 
 module.exports = function ( chai, utils ) {
 
@@ -14,7 +14,8 @@ module.exports = function ( chai, utils ) {
         options = typeof options !== "undefined" ? options : {};
         additionalRuleProps = typeof additionalRuleProps !== "undefined" ? additionalRuleProps : { reportFailures: true };
 
-        const ast = parser.parse( grammar );
+        const session = new Session( { grammar } );
+        const ast = session.parse( grammar );
 
         if ( ! options.allowedStartRules ) {
 
@@ -26,7 +27,7 @@ module.exports = function ( chai, utils ) {
 
         ast.rules = ast.rules.map( rule => Object.assign( rule, additionalRuleProps ) );
 
-        utils.flag( this, "object" )( ast, options );
+        utils.flag( this, "object" )( ast, session, options );
 
         new Assertion( ast ).like( props );
 
@@ -36,7 +37,8 @@ module.exports = function ( chai, utils ) {
 
         options = typeof options !== "undefined" ? options : {};
 
-        const ast = parser.parse( grammar );
+        const session = new Session( { grammar } );
+        const ast = session.parse( grammar );
 
         if ( ! options.allowedStartRules ) {
 
@@ -50,7 +52,7 @@ module.exports = function ( chai, utils ) {
 
         try {
 
-            utils.flag( this, "object" )( ast, options );
+            utils.flag( this, "object" )( ast, session, options );
             passed = true;
 
         } catch ( e ) {