feat: Maps page update

package.json

@@ -23,5 +23,6 @@
     "@types/react": "18.2.61",
     "typescript": "^4.9.5"
   },
-  "license": "CC-BY-4.0"
-}
+  "license": "CC-BY-4.0",
+  "packageManager": "yarn@1.22.22"
+}

pages/book/contracts.mdx

@@ -46,7 +46,7 @@ contract Example {
 }
 ```
 
-State variables must have a default value or initialized in [`init(){:tact}`](#init-function) function, that runs on deployment of the contract.
+State variables must have a default value or initialized in [`init(){:tact}`](#init-function) function, that runs on deployment of the contract. The only exception is persistent state variables of type [`map<k, v>{:tact}`](/book/maps) since they are initialized empty by default.
 
 <Callout>
 

pages/book/maps.mdx

@@ -2,42 +2,290 @@
 
 import { Callout } from 'nextra/components'
 
-The type `map<k, v>{:tact}` is used as a way to associate keys of type `k` with corresponding values of type `v`.
+The [composite type](/book/types#composite-types) `map<k, v>{:tact}` is used as a way to associate keys of type `k` with corresponding values of type `v`.
 
-Possible key types:
+For example, `map<Int, Int>{:tact}` uses [`Int{:tact}`][int] type for its keys and values:
 
-* [`Int{:tact}`][ints]
-* `Address{:tact}`
+```tact
+struct IntToInt {
+    counters: map<Int, Int>;
+}
+```
+
+## Allowed types
 
-Possible value types:
+Allowed key types:
 
-* [`Int{:tact}`][ints]
+* [`Int{:tact}`][int]
+* [`Address{:tact}`][p]
+
+Allowed value types:
+
+* [`Int{:tact}`][int]
 * [`Bool{:tact}`](/book/types#booleans)
-* `Cell{:tact}`
-* `Address{:tact}`
+* [`Cell{:tact}`][p]
+* [`Address{:tact}`][p]
 * [Struct](/book/structs-and-messages#structs)
 * [Message](/book/structs-and-messages#messages)
 
-For example, `map<Int, Int>{:tact}` uses [`Int{:tact}`][ints] type for its keys and values:
+## Operations
+
+### Declare
+
+As a [local variable](/book/statements#let), using `emptyMap(){:tact}` function of standard library:
 
 ```tact
-struct IntToInt {
-    counters: map<Int, Int>;
+let fizz: map<Int, Int> = emptyMap();
+let fizz: map<Int, Int> = null; // identical to the previous line, but less descriptive
+```
+
+As a [persistent state variables](/book/contracts#variables):
+
+```tact
+contract Example {
+    fizz: map<Int, Int>; // Int keys to Int values
+    init() {
+        self.fizz = emptyMap(); // redundant and can be removed!
+    }
+}
+```
+
+Note, that [persistent state variables](/book/contracts#variables) of type `map<k, v>{:tact}` are initialized empty by default and don't need default values or an initialization in the [`init(){:tact}` function](/book/contracts#init-function).
+
+### Set values, `.set()` [#set]
+
+To set or replace the value under a key call the `.set(){:tact}` [method](/book/functions#extension-function), which is accessible for all maps.
+
+```tact
+// Empty map
+let fizz: map<Int, Int> = emptyMap();
+
+// Setting a couple of values under different keys
+fizz.set(7, 7);
+fizz.set(42, 42);
+
+// Overriding one of the existing key-value pairs
+fizz.set(7, 68); // key 7 now points to value 68
+```
+
+### Get values, `.get()` [#get]
+
+To check if a key is found in the map by calling the `.get(){:tact}` [method](/book/functions#extension-function), which is accessible for all maps. This will return `null{:tact}` if the key is missing, or the value if the key is found.
+
+```tact
+// Empty map
+let fizz: map<Int, Int> = emptyMap();
+
+// Setting a value
+fizz.set(68, 0);
+
+// Getting the value by its key
+let gotButUnsure: String? = fizz.get(68);          // returns String or null, therefore the type is String?
+let mustHaveGotOrErrored: String = fizz.get(68)!!; // explicitly asserting that the value must not be null,
+                                                   // which may crush at runtime if the value is, in fact, null
+
+// Alternatively, we can check for the key in the if statement
+if (gotButUnsure != null) {
+    // Hooray, let's use !! without fear now and cast String? to String
+    let definitelyGotIt: String = fizz.get(68)!!;
+} else {
+    // Do something else...
+}
+```
+
+### Delete entries [#del]
+
+To delete a single key-value pair (single entry), simply assign the `null{:tact}` value to the key when using the [`.set(){:tact}`](#set) [method](/book/functions#extension-function).
+
+```tact
+// Empty map
+let fizz: map<Int, Int> = emptyMap();
+
+// Setting a couple of values under different keys
+fizz.set(7, 123);
+fizz.set(42, 321);
+
+// Deleting one of the keys
+fizz.set(7, null); // the entry under key 7 is now deleted
+```
+
+To delete all the entries from the map, re-assign the map using the `emptyMap(){:tact}` function:
+
+```tact
+// Empty map
+let fizz: map<Int, Int> = emptyMap();
+
+// Setting a couple of values under different keys
+fizz.set(7, 123);
+fizz.set(42, 321);
+
+// Deleting all of the entries at once
+fizz = emptyMap();
+fizz = null; // identical to the previous line, but less descriptive
+```
+
+With this approach all previous entries of the map are completely discarded from the contract even if the map was declared as its persistent state variable. As a result, assigning maps to `emptyMap(){:tact}` **does not** inflict any hidden or sudden [storage fees](https://docs.ton.org/develop/smart-contracts/fees#storage-fee).
+
+### Convert to a `Cell`, `.asCell()` [#convert]
+
+Use `.asCell(){:tact}` [method](/book/functions#extension-function) on maps to convert all their values to a [`Cell{:tact}`][p] type. Be mindful, that [`Cell{:tact}`][p] type is able to store up to 1023 bits, so converting larger maps to the Cell will result in error.
+
+As an example, this method is useful for sending small maps directly in the body of the reply:
+
+```tact
+contract Example {
+    // Persistent state variables
+    fizz: map<Int, Int>; // our map
+
+    // Constructor (initialization) function of the contract
+    init() {
+        // Setting a bunch of values
+        self.fizz.set(0, 3);
+        self.fizz.set(1, 14);
+        self.fizz.set(2, 15);
+        self.fizz.set(3, 926);
+        self.fizz.set(4, 5_358_979_323_846);
+    }
+
+    // Internal message receiver, which responds to empty messages
+    receive() {
+        // Here we're converting the map to a Cell and making a reply with it
+        self.reply(self.fizz.asCell());
+    }
 }
 ```
 
-Additionally, maps allow [integer serialization](/book/integers#serialization-types) of their keys, values or both to [preserve space and reduce storage costs](/book/integers#serialization):
+### Traverse over entries [#traverse]
+
+At the moment Tact doesn't have a special syntax for iterating over maps. However, it's possible to use maps as a simple arrays if you define a `map<Int, v>{:tact}` with an [`Int{:tact}`][int] type for the keys and keep track of the number of items in the separate variable:
+
+```tact
+contract Iteration {
+    // Persistent state variables
+    counter: Int as uint32;    // counter of map entries, serialized as a 32-bit unsigned
+    record: map<Int, Address>; // Int to Address map
+
+    // Constructor (initialization) function of the contract
+    init() {
+        self.counter = 0; // Setting the self.counter to 0
+    }
+
+    // Internal message receiver, which responds to a String message "Add"
+    receive("Add") {
+        // Get the Context Struct
+        let ctx: Context = context();
+        // Set the entry: counter Int as a key, ctx.sender Address as a value
+        self.record.set(self.counter, ctx.sender);
+        // Increase the counter
+        self.counter += 1;
+    }
+
+    // Internal message receiver, which responds to a String message "Send"
+    receive("Send") {
+        // Loop until the value of self.counter (over all the self.record entries)
+        let i: Int = 0; // declare usual i for loop iterations
+        while (i < self.counter) {
+           send(SendParameters{
+                bounce: false,              // do not bounce back this message
+                to: self.record.get(i)!!,   // set the sender address, knowing that key i exists in the map
+                value: ton("0.0000001"),    // 100 nanoToncoins (nano-tons)
+                mode: SendIgnoreErrors,     // send ignoring errors in transaction, if any
+                body: "SENDING".asComment() // String "SENDING" converted to a Cell as a message body
+            });
+            i += 1; // don't forget to increase the i
+        }
+    }
+
+    // Getter function for obtaining the value of self.record
+    get fun map(): map<Int, Address> {
+        return self.record;
+    }
+
+    // Getter function for obtaining the value of self.counter
+    get fun counter(): Int {
+        return self.counter;
+    }
+}
+```
+
+It's often useful to set an upper-bound restriction on such maps, so that you [don't hit the limits](#limits-and-drawbacks).
+
+<Callout type="warning" emoji="⚠️">
+
+  Note, that manually keeping track of number of items or checking the length of such map is very error-prone and generally discouraged. Instead, try to wrap your map into the [Struct](/book/structs-and-messages#structs) and define [extension functions](/book/functions#extension-function) on it. See example in the Cookbook: [How to emulate an array using a map wrapped in a Struct](/cookbook#how-to-emulate-an-array-using-a-map-wrapped-in-a-struct).
+
+</Callout>
+
+<Callout>
+
+  This example was adapted from [howardpen9/while-example-tact](https://github.com/howardpen9/while-example-tact/blob/de5807fcd20dba5f6a3748d112511477fb22bfcc/contracts/awesome.tact#L19C10-L19C10).
+
+  See other examples of map usage in the Cookbook:\
+  [How to emulate a stack using a map wrapped in a Struct](/cookbook#how-to-emulate-a-circular-buffer-using-a-map-wrapped-in-a-struct)\
+  [How to emulate a circular buffer using a map wrapped in a Struct](/cookbook#how-to-emulate-a-circular-buffer-using-a-map-wrapped-in-a-struct)
+
+</Callout>
+
+## Serialization
+
+It's possible to do [integer serialization](/book/integers#serialization-types) of map keys, values or both to [preserve space and reduce storage costs](/book/integers#serialization):
 
 ```tact
 struct SerializedMapInside {
+    // Both keys and values here would be serialized as 8-bit unsigned integers,
+    // thus preserving the space and reducing storage costs:
     countersButCompact: map<Int as uint8, Int as uint8>;
 }
 ```
 
 <Callout>
 
-  Read about other serialization options: [Compatibility with FunC](/book/func#convert-serialization)
+  Read about other serialization options: [Compatibility with FunC](/book/func#convert-serialization).
 
 </Callout>
 
-[ints]: /book/integers
+## Limits and drawbacks
+
+While maps can be convenient to work with on a small scale, they cause a number of issues if the number of items is unbounded and map can significantly grow in size:
+
+* As the upper bound of the smart contract state size is around $65\,000$ items of type [`Cell{:tact}`][p], it constrains the storage limit of maps to be about $30\,000$ key-value pairs for the whole contract.
+
+* The more entries you have in a map, the bigger [compute fees](https://docs.ton.org/develop/howto/fees-low-level#computation-fees) you'll get. Thus, working with large maps makes compute fees tough to predict and manage.
+
+* Using a large map in a single contract doesn't allow to distribute its workload. Hence, it can make the overall performance much worse compared to  using a smaller map and a bunch of interacting smart contracts.
+
+To resolve such issues you can set an upper-bound restriction on a map as a constant and check against it every time you're setting a new value to the map:
+
+```tact
+contract Example {
+    // Declare a compile-time constant upper-bound for our map
+    const MaxMapSize: Int = 42;
+
+    // Persistent state variables
+    arr: map<Int, Int>; // "array" of String values as a map
+    arrLength: Int = 0;    // length of the "array", defaults to 0
+
+    // Constructor (initialization) function of the contract
+    init() {}
+
+    // Internal function for pushing an item to the end of the "array"
+    fun arrPush(item: String) {
+        if (self.arrLength >= self.MaxMapSize) {
+            // Do something, stop the operation, for example
+        } else {
+            // Proceed with adding new item
+            self.arr.set(self.arrLength, item);
+            self.arrLength += 1;
+        }
+    }
+}
+```
+
+If you still need a large map or an unbound (infinitely large) map, it's better to architect your smart contracts according to the [asynchronous and actor-based model of TON blockchain](https://docs.ton.org/learn/overviews/ton-blockchain). That is, to use contract sharding and essentially make the whole blockchain a part of your map(s).
+
+{/*
+  TODO: Add reference to sharding page as per: https://github.com/tact-lang/tact-docs/issues/155
+*/}
+
+[int]: /book/integers
+[p]: /book/types#primitive-types