feat: Expressions page (#136)

Plus:
* initOf got cross-linked correctly
* various smaller fixes related to literals

pages/book/_meta.js

@@ -10,6 +10,7 @@ export default {
   types: 'Type system overview',
   integers: 'Integers',
   operators: 'Operators',
+  expressions: 'Expressions',
   statements: 'Statements',
   constants: 'Constants',
   functions: 'Functions',

pages/book/composite-types.mdx

@@ -4,7 +4,7 @@ import { Callout } from 'nextra/components'
 
 Tact supports a number of [primitive data types](/book/types#primitive-types) that are tailored for smart contract use. However, using individual means of storage often becomes cumbersome, so there are two main ways to combine multiple primitives together: [Structs](#structs) and [Messages](#messages).
 
-Note, while Traits and Contracts are also considered a part of the Tacts type system, one can't pass them around like [Structs](#structs) or [Messages](#messages). Instead, one can obtain the initial state of the given Contract by using the [initOf](/book/statements#initof) statement described later in the Book.
+Note, while Traits and Contracts are also considered a part of the Tacts type system, one can't pass them around like [Structs](#structs) or [Messages](#messages). Instead, one can obtain the initial state of the given Contract by using the [`initOf{:tact}`](/book/expressions#initof) expression.
 
 <Callout type="warning" emoji="⚠️">
 

pages/book/expressions.mdx

@@ -0,0 +1,193 @@
+# Expressions
+
+import { Callout } from 'nextra/components'
+
+Every operator in Tact forms an expression, but there's much more to uncover as Tact offers a wide range of expressive options to choose from.
+
+## Literals
+
+Literals represent values in Tact. These are fixed values—not variables—that you _literally_ provide in your code. All literals in Tact are expressions themselves.
+
+You can also call [extension functions](/book/functions#extension-function) defined on certain [primitive types][p] corresponding to literals right on the literal values:
+
+```tact
+// Calling toString() defined for Int on a integer literal:
+42.toString();
+
+// Calling asComment() defined for String on a string literal:
+"Tact is awesome!".asComment();
+```
+
+### Integer literals
+
+Integer literals can be written in [decimal](/book/integers#decimal) (base $10$), [hexadecimal](/book/integers#hexadecimal) (base $16$), [octal](/book/integers#octal) (base $8$) and [binary](/book/integers#binary) (base $2$) notations:
+
+* A [_decimal_ integer](/book/integers#decimal) literal is a sequence of digits ($\mathrm{0 - 9}$).
+
+* A leading $\mathrm{0x}$ (or $\mathrm{0X}$) indicates a [hexadecimal integer](/book/integers#hexadecimal) literal. They can include digits ($\mathrm{0 - 9}$) and the letters $\mathrm{a - f}$ and $\mathrm{A - F}$. Note, that the case of a character does not change its value. Therefore: $\mathrm{0xa}$ = $\mathrm{0xA}$ = $10$ and $\mathrm{0xf}$ = $\mathrm{0xF}$ = $15$.
+
+* A leading $\mathrm{0o}$ (or $\mathrm{0O}$) indicates a [octal integer](/book/integers#octal) literals. They can include only the digits $\mathrm{0 - 7}$.
+
+* A leading $\mathrm{0b}$ (or $\mathrm{0B}$) indicates a [binary integer](/book/integers#binary) literal. THey can only include the digits $0$ and $1$.
+
+<Callout type="warning" emoji="⚠️">
+  Be wary that in Tact integer literals with a leading $0$ are still considered decimals, unlike in JavaScript/TypeScript where leading $0$ indicates an octal!
+</Callout>
+
+Some examples of integer literals:
+
+```tact
+// decimal, base 10:
+0, 42, 1_000, 020
+
+// hexadecimal, base 16:
+0xABC, 0xF, 0x0011
+
+// octal, base 8:
+0o777, 0o001
+
+// binary, base 2:
+0b01111001_01101111_01110101_00100000_01100001_01110010_01100101_00100000_01100001_01110111_01100101_01110011_01101111_01101101_01100101
+```
+
+Read more about integers and [`Int{:tact}`](/book/integers) type on the dedicated page: [Integers](/book/integers).
+
+### Boolean literals
+
+The [`Bool{:tact}`](/book/types#booleans) type has only two literal values: `true{:tact}` and `false{:tact}`.
+
+```tact
+true == true;
+true != false;
+```
+
+Read more about booleans and [`Bool{:tact}`](/book/types#booleans) type in the dedicated chapter: [Booleans](/book/types#booleans).
+
+### String literals
+
+A string literal is zero or more characters enclosed in double (`"`) quotation marks. All string literals are objects of [`String{:tact}`][p] type.
+
+```tact
+"foo"
+"1234"
+
+// Note, that at the moment Tact strings can't have escape characters in them:
+"line \n another"; // SYNTAX ERROR!, see: https://github.com/tact-lang/tact/issues/25
+
+// This means, that double quotes inside strings are also prohibited:
+"this \"can't be!\""; // SYNTAX ERROR!
+```
+
+Read more about strings and [`String{:tact}`][p] type there: [Primitive types][p].
+
+### `null` literal
+
+The `null{:tact}` value is written with a `null{:tact}` literal. It's **not** an [identifier](#identifiers) and doesn't refer to any object. It's also **not** an instance of a [primitive type][p]. Instead, `null{:tact}` represents a lack of identification and the intentional absence of any value.
+
+```tact
+let var: Int? = null; // variable, which can hold null value
+var = 42;
+if (var != null) {
+    var!! + var!!;
+}
+```
+
+Read more about working with `null{:tact}` in the dedicated chapter: [Optionals](/book/composite-types#optionals).
+
+## Identifiers
+
+An identifier is a sequence of characters in the code that _identifies_ a [variable](/book/statements#let), [constant](/book/constants), [map](/book/types#maps) and a [function](/book/functions), as well as a [Struct][s], [Message][m], [contract](/book/types#contract), [trait](/book/types#trait), or their fields and methods. Identifiers are case-sensitive and not quoted.
+
+In Tact, identifiers can contain latin lowercase letters (`a-z`), latin uppercase letters (`A-Z`), underscores (`_`) and digits ($\mathrm{0 - 9}$), but may not start with a digit. An identifier differs from a [string](#string-literals) in that a string is data, while an identifier is part of the code. 
+
+Note, that when identifiers for [primitive types][p] start with an uppercase letter. Used-defined [composite types](/book/composite-types), such as [Structs][s] and [Messages][m] also must be capitalized.
+
+## Instantiation
+
+You can create instances of the following types:
+
+* [Structs][s]
+* [Messages][m]
+
+```tact
+struct StExample {
+    fieldInit: Int = 1;
+    fieldUninit: Int;
+}
+
+fun example() {
+    StExample{ fieldUninit: 2 };               // instance with default value of fieldInit
+    StExample{ fieldInit: 0, fieldUninit: 2 }; // instance with both fields set
+}
+```
+
+## Field access
+
+You can directly access fields of the following types:
+
+* [Structs][s]
+* [Messages][m]
+
+```tact
+struct StExample {
+    fieldInit: Int = 1;
+    fieldUninit: Int;
+}
+
+fun example(): Int {
+    let struct: StExample = StExample{ fieldUninit: 2 }; // instantiation
+
+    struct.fieldInit;          // access a field
+    return struct.fieldUninit; // return field value from the function
+}
+```
+
+## Extension function call
+
+[Extension functions](/book/functions#extension-function) are defined only on specific types. They can be called similar to method calls in many other languages:
+
+```tact
+42.toString(); // toString() is a stdlib function that is defined on Int type
+```
+
+## Static function call
+
+Anywhere in the function body, a global [static function](/book/functions#global-static-functions) or an internal function of a contract can be called:
+
+```tact
+contract ExampleContract {
+    init() {}
+    receive() {
+        now(); // now() is a static function of stdlib
+        let expiration: Int = now() + 1000; // operation and variable declaration
+        expiration = self.answerQuestion(); // internal function
+    }
+    fun answerQuestion(): Int {
+        return 42;
+    }
+}
+```
+
+## `initOf`
+
+Expression `initOf{:tact}` computes initial state (`StateInit{:tact}`) of a [contract](/book/types#contracts):
+
+```tact
+//                     argument values for the init() function of the contract
+//                     ↓   ↓
+initOf ExampleContract(42, 100); // returns a Struct StateInit{}
+//     ---------------
+//     ↑
+//     name of the contract
+```
+
+Where `StateInit{:tact}` is a built-in [Struct][s], that consists of:
+
+Field  | Type               | Description
+:----- | :----------------- | :----------
+`code` | [`Cell{:tact}`][p] | initial code of the contract (the compiled bytecode)
+`data` | [`Cell{:tact}`][p] | initial data of the contract (arguments of `init(){:tact}` function of the contract)
+
+[p]: /book/types#primitive-types
+[s]: /book/composite-types#structs
+[m]: /book/composite-types#messages

pages/book/integers.mdx

@@ -9,7 +9,7 @@ It's capable of storing integers between $-2^{256}$ and $2^{256} - 1.$
 
 ## Notation
 
-Tact supports various ways of writing primitive values of `Int{:tact}` (integer literals).
+Tact supports various ways of writing primitive values of `Int{:tact}` as [integer literals](/book/expressions#integer-literals).
 
 Most of the notations allow adding underscores (`_`) in-between digits, except for:
 * Representations in strings, as seen in [nano-tons](#nano-tons) case.

pages/book/send.mdx

@@ -55,7 +55,7 @@ send(SendParameters{
 
 ## Deploy contract
 
-To deploy a contract you need to calculate its address and init package, then send it with an initial message. You can always send an init package with a message and it would be ignored, but would cost more than the message without an init package.
+To deploy a contract you need to calculate its address and initial state with [`initOf{:tact}`](/book/expressions#initof), then send them in the initialization message:
 
 ```tact
 let init: StateInit = initOf SecondContract(arg1, arg2);
@@ -70,4 +70,4 @@ send(SendParameters{
     data: init.data,
     body: "Hello, World!".asComment()
 });
-```
+```

pages/book/types.mdx

@@ -110,6 +110,8 @@ Learn more about them on a dedicated page about [composite types](/book/composit
 
 Contracts are the main entry of a smart contract on the TON blockchain. It holds all [functions](/book/functions), [getters](/book/functions#getter-functions), and [receivers](/book/functions#receiver-functions) of a TON contract.
 
+Contracts and [traits](#traits) have a built-in [identifier](/book/expressions#identifiers) `self{:tact}`, which is used for referring to their fields (persistent state variables) and methods (internal functions).
+
 ```tact
 // Basic example of a counter contract:
 contract HelloWorld {