Support ES6 syntax for Import statements

[sheetalkamat]

As per ES6 grammer

ImportDeclaration :
import ImportClause FromClause ;
import ModuleSpecifier ;

ImportClause :
ImportedDefaultBinding
NameSpaceImport
NamedImports
ImportedDefaultBinding , NameSpaceImport
ImportedDefaultBinding , NamedImports

ImportedDefaultBinding :
ImportedBinding

NameSpaceImport :
* as ImportedBinding

NamedImports :
{ }
{ ImportsList }
{ ImportsList , }

FromClause :
from ModuleSpecifier

ImportsList :
ImportSpecifier
ImportsList , ImportSpecifier

ImportSpecifier :
ImportedBinding
IdentifierName as ImportedBinding

ModuleSpecifier :
StringLiteral

ImportedBinding :
BindingIdentifier

Downlevel emit and meaning of each import syntax:

import BindingIdentifier from StringLiteral
//import _tmp = require(StringLiteral)
//BindingIdentifier = _tmp.default

import * as BindingIdentifier from StringLiteral
//import BindingIdentifier = require(“StringLiteral”)

import { } from StringLiteral
//import _tmp = require(StringLiteral)

import { ImportedBinding1, … } from StringLiteral
//import _tmp = require(StringLiteral)
//ImportedBinding1= _tmp.importedBinding1
//… 

import { IdentifierName1 as ImportedBinding1, … } from StringLiteral
//import _tmp = require(StringLiteral)
//ImportedBinding1 = _tmp. IdentifierName1
//…

import BindingIdentifier1, * as BindingIdentifier2 from StringLiteral
//import BindingIdentifier2 = require(StringLiteral)
//BindingIdentifier1 =  BindingIdentifier2.default
//…

Import BindingIdentifier1, { ImportedBinding2,  IdentifierName1 as ImportedBinding3, … } from StringLiteral
//import _tmp = require(StringLiteral)
//BindingIdentifier1 = _tmp.default
//ImportedBinding2 = _tmp. ImportedBinding1
//ImportedBinding3 = _tmp. IdentifierName1
//… 

In all cases, the import syntax when the import syntax is specified it has exactly the meaning of commented code. The import bindings will have exact same meaning as that of declaration it binds to and there will be no emit if the binding corresponds to type or uninitialized module

[rbuckton]

ES6 Imported bindings are 'mutable', meaning that the module exporting the binding can update the value at any time, and any consumers of the module should have the new value.

The downlevel syntax below creates new identifiers that copy the value of the export, which would break that assumption.

A better way might be:

import { ImportedBinding1 } from StringLiteral;
console.log(ImportedBinding1);

// import _tmp = require(StringLiteral);
// console.log(_tmp.ImportedBinding1);

import { IdentifierName1 as ImportedBinding1 } from StringLiteral;
console.log(ImportedBinding1);

// import _tmp = require(StringLiteral);
// console.log(_tmp.IdentifierName1);

Keeping a reference to the imported module and always pointing to the actual export binding ensures this works as expected.

Sent from my Windows Phone

---

From: Sheetal Nandimailto:notifications@github.com
Sent: ý11/ý18/ý2014 5:29 PM
To: Microsoft/TypeScriptmailto:TypeScript@noreply.github.com
Subject: [TypeScript] Support ES6 syntax for Import statements (#1203)

As per ES6 grammer

ImportDeclaration :
import ImportClause FromClause ;
import ModuleSpecifier ;

ImportClause :
ImportedDefaultBinding
NameSpaceImport
NamedImports
ImportedDefaultBinding , NameSpaceImport
ImportedDefaultBinding , NamedImports

ImportedDefaultBinding :
ImportedBinding

NameSpaceImport :

• as ImportedBinding

NamedImports :
{ }
{ ImportsList }
{ ImportsList , }

FromClause :
from ModuleSpecifier

ImportsList :
ImportSpecifier
ImportsList , ImportSpecifier

ImportSpecifier :
ImportedBinding
IdentifierName as ImportedBinding

ModuleSpecifier :
StringLiteral

ImportedBinding :
BindingIdentifier

Downlevel emit and meaning of each import syntax:

import BindingIdentifier from StringLiteral
//import _tmp = require(StringLiteral)
//BindingIdentifier = _tmp.default

import * as BindingIdentifier from StringLiteral
//import BindingIdentifier = require(“StringLiteral”)

import { } from StringLiteral
//import _tmp = require(StringLiteral)

import { ImportedBinding1, … } from StringLiteral
//import _tmp = require(StringLiteral)
//ImportedBinding1= _tmp.importedBinding1
//…

import { IdentifierName1 as ImportedBinding1, … } from StringLiteral
//import _tmp = require(StringLiteral)
//ImportedBinding1 = _tmp. IdentifierName1
//…

import BindingIdentifier1, * as BindingIdentifier2 from StringLiteral
//import BindingIdentifier2 = require(StringLiteral)
//BindingIdentifier1 = BindingIdentifier2.default
//…

Import BindingIdentifier1, { ImportedBinding2, IdentifierName1 as ImportedBinding3, … } from StringLiteral
//import _tmp = require(StringLiteral)
//BindingIdentifier1 = _tmp.default
//ImportedBinding2 = _tmp. ImportedBinding1
//ImportedBinding3 = _tmp. IdentifierName1
//…

In all cases, the import syntax when the import syntax is specified it has exactly the meaning of commented code. The import bindings will have exact same meaning as that of declaration it binds to and there will be no emit if the binding corresponds to type or uninitialized module

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/1203.

[sheetalkamat]

According to spec section 8.1.1.5.3 the import bindings are immutable:

8.1.1.5.3   CreateImportBinding (N, M, N2)
The concrete Environment Record method CreateImportBinding for module environment records creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this environment record for N. M is a Module Record (see 15.2.1.15), and N2 is the name of a binding that exists in M’s module environment record. Access to the value of the new binding will indirectly access the bound value of value of thetarget binding.
1.  Let envRec be the module environment record for which the method was invoked.
2.  Assert: envRec does not already have a binding for N.
3.  Assert: M is a Module Record.
4.  Assert: M.[[ Environment]] has a binding for N2.
5.  Create an immutable indirect binding in envRec for N that references M and N2 as its target binding. Record that the is initialized.

[sheetalkamat]

But in example
a.ts

export var a = 10;

b.ts

import  {a} from "a"

a should result in dynamic value of a so we would need to emit the _tmp.Idenfitier kind of bindings. Agreed.

But it should also be error to do a = someexpression

[rbuckton]

Agreed. The bindings are only mutable on the export side