# Parser

The sourcecode for the TypeScript parser is located entirely in `parser.ts`. Scanner is *controlled* internally by the `Parser` to convert the source code to an AST. Here is a review of what the desired outcome is.

```
SourceCode ~~ scanner ~~> Token Stream ~~ parser ~~> AST
```

The parser is implemented as a singleton (similar reasons to `scanner`, don't want to recreate it if we can reinit it). It is actually implemented as `namespace Parser` which contains *state* variables for the Parser as well as a singleton `scanner`. As mentioned before it contains a `const scanner`. The parser functions manage this scanner.

## Usage by program

Parser is driven indirectly by Program (indirectly as its actually by `CompilerHost` which we mentioned previously). Basically this is the simplified call stack:

```
Program ->
    CompilerHost.getSourceFile ->
        (global function parser.ts).createSourceFile ->
            Parser.parseSourceFile
```

The `parseSourceFile` not only primes the state for the Parser but also primes the state for the `scanner` by calling `initializeState`. It then goes on to parse the source file using `parseSourceFileWorker`.

## Sample Usage

Before we dig too deep into the parser internals, here is a sample code that uses the TypeScript's parser to get the AST of a source file (using `ts.createSourceFile`), and then print it.

`code/compiler/parser/runParser.ts`

```typescript
import * as ts from "ntypescript";

function printAllChildren(node: ts.Node, depth = 0) {
    console.log(new Array(depth + 1).join('----'), ts.formatSyntaxKind(node.kind), node.pos, node.end);
    depth++;
    node.getChildren().forEach(c=> printAllChildren(c, depth));
}

var sourceCode = `
var foo = 123;
`.trim();

var sourceFile = ts.createSourceFile('foo.ts', sourceCode, ts.ScriptTarget.ES5, true);
printAllChildren(sourceFile);
```

This will print out the following:

```typescript
SourceFile 0 14
---- SyntaxList 0 14
-------- VariableStatement 0 14
------------ VariableDeclarationList 0 13
---------------- VarKeyword 0 3
---------------- SyntaxList 3 13
-------------------- VariableDeclaration 3 13
------------------------ Identifier 3 7
------------------------ FirstAssignment 7 9
------------------------ FirstLiteralToken 9 13
------------ SemicolonToken 13 14
---- EndOfFileToken 14 14
```

This looks like a (very right sided) tree if you tilt your head to the left.