Drop v8 vm for native implementation, update tests accordingly 🚀.

- Implements and closes #1 by @edmulraney.
- Better performance & browser support.

index.js

@@ -1,75 +1,42 @@
-const vm = require('vm')
-
-module.exports = class Guards {
+class Guards {
   constructor () {
     this.guardRegex = /[^=<>!']=[^=]/
 
     this.templateRegex = /[^|]\|[^|]/
-
-    this.optionsVM = {
-      displayErrors: true,
-      filename: 'guards'
-    }
   }
 
-  getStack () {
-    const origin = Error.prepareStackTrace
-    const error = new Error()
-
-    Error.prepareStackTrace = (_, stack) => stack
-
-    Error.captureStackTrace(error, this.getStack)
-
-    const stack = error.stack
-
-    Error.prepareStackTrace = origin
-
-    // V8 stack traces.
-    return stack
+  buildPredicate (constants, guard) {
+    return new Function(
+      '',
+      [
+        `const { ${this.destructureProps(constants)} } = ${JSON.stringify(constants)}`,
+        `return ${guard}`
+      ].join(';')
+    )
   }
 
-  equal (constants) {
-    return template => {
-      const guards = this.parse(template.raw[0])
-      const lineOffset = this.getStack()[1].getLineNumber()
-      const firstTruthyGuard = (
-        guards.map(
-          g => this.runInVM(g.eval, constants, lineOffset)
-        )
-      ).findIndex(a => a === true)
-
-      if (firstTruthyGuard === -1) this.error(`Non-exhaustive patterns in guards at line ${lineOffset}!`)
-
-      // First truthy guard is returned, like in Haskell.
-      return guards[firstTruthyGuard].result
-    }
+  destructureProps (constants) {
+    return Object.keys(constants).join(',')
   }
 
-  error (e) {
-    console.error(e)
-
-    process.exit(1)
-  }
+  equal (constants) {
+    const self = this
 
-  parse (template) {
     // Inline guards need filtering.
-    return template
+    return template => template.raw[0]
       .split(this.templateRegex)
       .filter(g => g.trim() !== '')
       .map((g, i) => {
         // Remove break line and extract the guard.
         const parts = g.trim().split(this.guardRegex)
 
-        return {
-          eval: parts[0],
-          result: JSON.parse(`${parts[1].trim().replace(/'/g, '"')}`)
-        }
+        return [
+          parts[0],
+          JSON.parse(`${parts[1].trim().replace(/'/g, '"')}`)
+        ]
       })
-  }
-
-  runInVM (code, sandbox, lineOffset) {
-    const options = Object.assign({}, this.optionsVM, { lineOffset: lineOffset })
-
-    return vm.runInNewContext(code, sandbox, options)
+      .find(g => self.buildPredicate(constants, g[0])())[1]
   }
 }
+
+module.exports = (c, t) => (new Guards()).equal(c, t)