feat(interpreter): allow passing external variables

fixes #174

src/__tests__/interpreter.test.js

@@ -460,6 +460,20 @@ describe('interpreter', () => {
     }
   )
 
+  it(`allow passing single variable`, () => {
+    expect(execute('$foo')(null, { foo: 3 })).toEqual(3)
+  })
+
+  it(`allow passing multiple variables`, () => {
+    expect(execute('$foo + $bar')(null, { foo: 4, bar: -2 })).toEqual(2)
+  })
+
+  it(`doesn't allow passing functions`, () => {
+    expect((): mixed =>
+      execute('foo 4')(null, { foo: (x: ?mixed): number => 3 })
+    ).toThrow()
+  })
+
   it('throws syntax error on invalid source code', () => {
     expect(() => {
       execute('¡')

src/interpreter.js

@@ -4,5 +4,11 @@ import compiler from './compiler'
 import type { OutputType, SourceCodeType } from './types'
 import builtIns from './builtins.js'
 
-export default (input: SourceCodeType): OutputType =>
-  eval(compiler(input))(builtIns) // eslint-disable-line no-eval
+export default (sourceCode: SourceCodeType): OutputType => {
+  const compiledFunction = eval(compiler(sourceCode)) // eslint-disable-line no-eval
+  return (input: mixed, variables?: {[string]: mixed}): mixed =>
+    compiledFunction({
+      ...builtIns,
+      ...JSON.parse(JSON.stringify(variables || {}))
+    })(input)
+}

src/types.js

@@ -194,7 +194,7 @@ export type ProjectableNodeType =
 
 export type AssignmentType = [PrimitiveNodeType, NodeType];
 export type AssignmentsType = Array<AssignmentType>;
-export type OutputType = mixed => mixed;
+export type OutputType = (input: mixed, variables?: {[string]: mixed}) => mixed;
 export type SourceCodeType = string;
 export type GeneratedCodeType = string;
 export type ParserType = SourceCodeType => ParserReturnValueType;