Eoin Davey.

- MSc. Mathematics @ Universiteit van Amsterdam.

- Former SRE @ Google.

- Éireannach.

v1.3.0 of *tsPEG* has just been released! Check it out on Github or install it with `npm install -g tspeg`

.

Below is a list of the latest new features:

As well as assigning parsing results to variables and storing them on the AST, *tsPEG* also allows you to create **computed properties**, which are fields on the AST that are computed when the parser is run.

Computed properties are added to a rule by appending a new expression after the rule description like

```
.<propertyname> = <type> { <code to calculate property> }
```

Let’s specify a small grammar to match simple addition expressions like “12+42”, “123+456” etc.

```
sum := left=num '\+' right=num
num := '[0-9]+'
```

We can add computed properties to this to compute the value of this sum at parse time, instead of writing our own function to do it after. First we add a computed property to `num`

to store the value of the number:

```
sum := left=num '\+' right=num
num := literal='[0-9]+'
.value = number { return parseInt(this.literal); }
```

As you can see we’ve assigned the text match to the field `literal`

, and added a computed property called `value`

which is the `literal`

field parsed as an integer.

Now we can add a computed property to `sum`

to do the arithmetic, this is very simple

```
sum := left=num '\+' right=num
.sum = number { return this.left.value + this.right.value }
num := literal='[0-9]+'
.value = number { return parseInt(this.literal); }
```

We use the computed property of the `num`

rule to compute our `sum`

property. Let’s see this in action! Let’s try and parse the string “240+100”.

As you can see the AST has a field called `sum`

with the correct value of 340 in it. The `left`

and `right`

also have their computed `value`

properties of 240 and 100.

The introduction of computed properties means that now you might want to import some types or functions into your parser. Luckily you can now specify a header in the file that will be inserted directly into the generated parser. Anything at the top of the grammar file between two lines of three dashes `---`

will be inserted straight into the generated parser, allowing you to write grammars like

```
---
import { myFunc, myType } from "./mypackage";
---
rule := hello='Hello World'
.value = myType { return myFunc(this.hello); }
```

*tsPEG* now supports a special match expression “`@`

” for storing the positions of matches. Using `@`

you can store the current position of the parser.

Returning to our addition of two numbers from earlier, which matches “12+56”, “123+456” etc.

```
sum := left=num '\+' right=num
num := '[0-9]+'
```

If we want to store the location of the “+” operator we can use the @ match like this:

```
sum := left=num pluspos=@ '\+' right=num
num := '[0-9]+'
```

This stores the position of the parser into the AST before the ‘+’ symbol, the generated interface for the AST looks like:

```
interface sum {
left: string;
pluspos: PosInfo;
right: string;
}
```