[zig/en] Introduce the Zig programming language.

author: Karchnu <karchnu@karchnu.fr> 2021-12-25 23:42:15 +0100
committer: Karchnu <karchnu@karchnu.fr> 2021-12-25 23:42:15 +0100
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+---
+language: "zig"
+filename: learnzig.zig
+contributors:
+    - ["Philippe Pittoli", "https://karchnu.fr/"]
+---
+
+
+[Zig][ziglang] aims to be a replacement for the C programming language.
+
+**WARNING**: this document expects you to understand a few basic concepts in computer science, such as pointers, stack and heap memory, etc.
+
+**WARNING**: Zig isn't considered as ready for production. Bugs are expected.
+DO NOT TRY ZIG AS YOUR FIRST PROGRAMMING EXPERIENCE.
+The compiler, even the language and its libraries aren't ready, yet.
+You've been warned.
+
+Prior knowledge of C is recommended.
+
+
+## Quick overview: Zig compared to C
+
+- Syntax is mostly the same, with some improvements (less ambiguity).
+- Zig introduces namespaces.
+- Try and catch mechanism, which is both convenient, efficient and optional.
+- Most of the C undefined behaviors (UBs) are fixed.
+- Raw pointers are safer to use and aren't nearly as used as before.
+  * The type system distinguishes between a pointer to a single value, or multiple values, etc.
+  * Slices are preferred, which is a structure with a pointer and a runtime known size, which characterizes most uses of pointers in the first place.
+- Some arbitrary language limitations are removed. For example, enumerations, structures and unions can have functions.
+- Simple access to SIMD operations (basic maths on vectors).
+- Zig provides both low-level features of C and the one provided through compiler extensions.
+  For example: packed structures.
+- An extensive standard library, including data structures and algorithms.
+- Cross-compilation capability is provided by default, without any dependency.
+  Different libc are provided to ease the process.
+  Cross-compilation works from, and to, any operating system and architecture.
+
+## Zig language
+
+
+```zig
+//! Top-level documentation.
+
+/// Documentation comment.
+
+// Simple comment.
+```
+
+
+### Hello world.
+```zig
+// Import standard library, reachable through the "std" constant.
+const std = @import("std");
+
+// "info" now refers to the "std.log.info" function.
+const info = std.log.info;
+
+// Usual hello world.
+// syntax: [pub] fn <function-name>(<arguments>) <return-type> { <body> }
+pub fn main() void {
+    // Contrary to C functions, Zig functions have a fixed number of arguments.
+    // In C: "printf" takes any number of arguments.
+    // In Zig: std.log.info takes a format and a list of elements to print.
+    info("hello world", .{});  // .{} is an empty anonymous tuple.
+}
+```
+
+### Booleans, integers and float.
+```zig
+// Booleans.
+// Keywords are prefered to operators for boolean operations.
+print("{}\n{}\n{}\n", .{
+    true and false,
+    true or false,
+    !true,
+});
+
+// Integers.
+const one_plus_one: i32 = 1 + 1;
+print("1 + 1 = {}\n", .{one_plus_one}); // 2
+
+// Floats.
+const seven_div_three: f32 = 7.0 / 3.0;
+print("7.0 / 3.0 = {}\n", .{seven_div_three}); // 2.33333325e+00
+
+// Integers have arbitrary value lengths.
+var myvar: u10 = 5; // 10-bit unsigned integer
+// Useful for example to read network packets, or complex binary formats.
+
+// Number representation is greatly improved compared to C.
+const one_billion = 1_000_000_000;         // Decimal.
+const binary_mask = 0b1_1111_1111;         // Binary. Ex: network mask.
+const permissions = 0o7_5_5;               // Octal.  Ex: Unix permissions.
+const big_address = 0xFF80_0000_0000_0000; // Hexa.   Ex: IPv6 address.
+
+
+// Overflow operators: tell the compiler when it's okay to overflow.
+var i: u8 = 0;  // "i" is an unsigned 8-bit integer
+i  -= 1;        // runtime overflow error (unsigned value always are positive)
+i -%= 1;        // okay (wrapping operator), i == 255
+
+// Saturation operators: values will stick to their lower and upper bounds.
+var i: u8 = 200;   // "i" is an unsigned 8-bit integer (values: from 0 to 255)
+i  +| 100 == 255   // u8: won't go higher than 255
+i  -| 300 == 0     // unsigned, won't go lower than 0
+i  *| 2   == 255   // u8: won't go higher than 255
+i <<| 8   == 255   // u8: won't go higher than 255
+```
+
+### Arrays.
+```zig
+// An array is a well-defined structure with a length attribute (len).
+
+// 5-byte array with undefined content (stack garbage).
+var array1: [5]u8 = undefined;
+
+// 5-byte array with defined content.
+var array2 = [_]u8{ 1, 2, 3, 4, 5 };
+// [_] means the compiler knows the length at compile-time.
+
+// 1000-byte array with defined content (0).
+var array3 = [_]u8{0} ** 1000;
+
+// Another 1000-byte array with defined content.
+// The content is provided by the "foo" function, called at compile-time and
+// allows complex initializations.
+var array4 = [_]u8{foo()} ** 1000;
+
+// In any case, array.len gives the length of the array,
+// array1.len and array2.len produce 5, array3.len and array4.len produce 1000.
+
+
+// Modifying and accessing arrays content.
+
+// Array of 10 32-bit undefined integers.
+var some_integers: [10]i32 = undefined;
+
+some_integers[0] = 30; // first element of the array is now 30
+
+var x = some_integers[0]; // "x" now equals to 30, its type is infered.
+var y = some_integers[1]; // Second element of the array isn't defined.
+                          // "y" got a stack garbage value (no runtime error).
+
+// Array of 10 32-bit undefined integers.
+var some_integers: [10]i32 = undefined;
+
+var z = some_integers[20]; // index > array size, compilation error.
+
+// At runtime, we loop over the elements of "some_integers" with an index.
+// Index i = 20, then we try:
+try some_integers[i]; // Runtime error 'index out of bounds'.
+                      // "try" keyword is necessary when accessing an array with
+                      // an index, since there is a potential runtime error.
+                      // More on that later.
+```
+
+### Multidimensional arrays.
+```zig
+
+const mat4x4 = [4][4]f32{
+    [_]f32{ 1.0, 0.0, 0.0, 0.0 },
+    [_]f32{ 0.0, 1.0, 0.0, 1.0 },
+    [_]f32{ 0.0, 0.0, 1.0, 0.0 },
+    [_]f32{ 0.0, 0.0, 0.0, 1.0 },
+};
+
+// Access the 2D array then the inner array through indexes.
+try expect(mat4x4[1][1] == 1.0);
+
+// Here we iterate with for loops.
+for (mat4x4) |row, row_index| {
+    for (row) |cell, column_index| {
+        // ...
+    }
+}
+```
+
+### Strings.
+```zig
+
+// Simple string constant.
+const greetings = "hello";
+// ... which is equivalent to:
+const greetings: *const [5:0]u8 = "hello";
+// In words: "greetings" is a constant value, a pointer on a constant array of 5
+// elements (8-bit unsigned integers), with an extra '0' at the end.
+// The extra "0" is called a "sentinel value".
+
+print("string: {s}\n", .{greetings});
+
+// This represents rather faithfully C strings. Although, Zig strings are
+// structures, no need for "strlen" to compute their size.
+// greetings.len == 5
+```
+
+### Slices.
+```zig
+
+// A slice is a pointer and a size, an array without compile-time known size.
+// Slices have runtime out-of-band verifications.
+
+const array = [_]u8{1,2,3,4,5};     // [_] = array with compile-time known size.
+const slice = array[0..array.len];  // "slice" represents the whole array.
+                                    // slice[10] gives a runtime error.
+```
+
+### Pointers.
+```zig
+
+// Pointer on a value can be created with "&".
+const x: i32 = 1;
+const pointer: *i32 = &x;  // "pointer" is a pointer on the i32 var "x".
+print("1 = {}, {}\n", .{x, pointer});
+
+// Pointer values are accessed and modified with ".*".
+if (pointer.* == 1) {
+    print("x value == {}\n", .{pointer.*});
+}
+
+// ".?" is a shortcut for "orelse unreachable".
+const foo = pointer.?; // Get the pointed value, otherwise crash.
+```
+
+### Optional values (?<type>).
+```zig
+// An optional is a value than can be of any type or null.
+
+// Example: "optional_value" can either be "null" or an unsigned 32-bit integer.
+var optional_value: ?u32 = null; // optional_value == null
+optional_value = 42;             // optional_value != null
+
+// "some_function" returns ?u32
+var x = some_function();
+if (x) |value| {
+    // In case "some_function" returned a value.
+    // Do something with 'value'.
+}
+```
+
+### Errors.
+```zig
+// Zig provides an unified way to express errors.
+
+// Errors are defined in error enumerations, example:
+const Error = error {
+    WatchingAnyNetflixTVShow,
+    BeOnTwitter,
+};
+
+// Normal enumerations are expressed the same way, but with "enum" keyword.
+const SuccessStory = enum {
+    DoingSport,
+    ReadABook,
+};
+
+
+// Error union (!).
+// Either the value "mylife" is an an error or a normal value.
+var mylife: Error!SuccessStory = Error.BeOnTwitter;
+// mylife is an error. Sad.
+
+mylife = SuccessStory.ReadABook;
+// Now mylife is an enum.
+
+
+// Zig ships with many pre-defined errors. Example:
+const value: anyerror!u32 = error.Broken;
+
+
+// Handling errors.
+
+// Some error examples.
+const Error = error {
+    UnExpected,
+    Authentication,
+};
+
+// "some_function" can either return an "Error" or an integer.
+fn some_function() Error!u8 {
+    return Error.UnExpected; // It returns an error.
+}
+
+// Errors can be "catch" without intermediate variable.
+var value = some_function() catch |err| switch(err) {
+    Error.UnExpected     => return err,   // Returns the error.
+    Error.Authentication => unreachable,  // Not expected. Crashes the program.
+    else                 => unreachable,
+};
+
+// An error can be "catch" without giving it a name.
+const unwrapped = some_function() catch 1234; // "unwrapped" = 1234
+
+// "try" is a very handy shortcut for "catch |err| return err".
+var value = try some_function();
+// If "some_function" fails, the current function stops and returns the error.
+// "value" can only have a valid value, the error already is handled with "try".
+```
+
+### Control flow.
+
+```zig
+// Conditional branching.
+
+if (condition) {
+    ...
+}
+else {
+    ...
+}
+
+// Ternary.
+var value = if (condition) x else y;
+
+// Shortcut for "if (x) x else 0"
+var value = x orelse 0;
+
+// If "a" is an optional, which may contain a value.
+if (a) |value| {
+    print("value: {}\n", .{value});
+}
+else {
+    print("'a' is null\n", .{});
+}
+
+// Get a pointer on the value (if it exists).
+if (a) |*value| { value.* += 1; }
+
+
+// Loops.
+
+// Syntax examples:
+//   while (condition) statement
+//   while (condition) : (end-of-iteration-statement) statement
+//
+//   for (iterable) statement
+//   for (iterable) |capture| statement
+//   for (iterable) statement else statement
+
+// Note: loops work the same way over arrays or slices.
+
+// Simple "while" loop.
+while (i < 10) { i += 1; }
+
+// While loop with a "continue expression"
+// (expression executed as the last expression of the loop).
+while (i < 10) : (i += 1) { ... }
+// Same, with a more complex continue expression (block of code).
+while (i * j < 2000) : ({ i *= 2; j *= 3; }) { ... }
+
+// To iterate over a portion of a slice, reslice.
+for (items[0..1]) |value| { sum += value; }
+
+// Loop over every item of an array (or slice).
+for (items) |value| { sum += value; }
+
+// Iterate and get pointers on values instead of copies.
+for (items) |*value| { value.* += 1; }
+
+// Iterate with an index.
+for (items) |value, i| { print("val[{}] = {}\n", .{i, value}); }
+
+// Iterate with pointer and index.
+for (items) |*value, i| { print("val[{}] = {}\n", .{i, value}); value.* += 1; }
+
+
+// Break and continue are supported.
+for (items) |value| {
+    if (value == 0)  { continue; }
+    if (value >= 10) { break;    }
+    // ...
+}
+
+// For loops can also be used as expressions.
+// Similar to while loops, when you break from a for loop,
+// the else branch is not evaluated.
+var sum: i32 = 0;
+// The "for" loop has to provide a value, which will be the "else" value.
+const result = for (items) |value| {
+    if (value != null) {
+        sum += value.?; // "result" will be the last "sum" value.
+    }
+} else 0;                  // Last value.
+```
+
+### Labels.
+```zig
+
+// Labels are a way to name an instruction, a location in the code.
+// Labels can be used to "continue" or "break" in a nested loop.
+outer: for ([_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }) |_| {
+    for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
+        count += 1;
+        continue :outer; // "continue" for the first loop.
+    }
+} // count = 8
+outer: for ([_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }) |_| {
+    for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
+        count += 1;
+        break :outer; // "break" for the first loop.
+    }
+} // count = 1
+
+
+// Labels can also be used to return a value from a block.
+var y: i32 = 5;
+const x = blk: {
+    y += 1;
+    break :blk y; // Now "x" equals 6.
+};
+// Relevant in cases like "for else" expression (explained in the following).
+
+// For loops can be used as expressions.
+// When you break from a for loop, the else branch is not evaluated.
+// WARNING: counter-intuitive.
+//      The "for" loop will run, then the "else" block will run.
+//      The "else" keyword has to be followed by the value to give to "result".
+//      See later for another form.
+var sum: u8 = 0;
+const result = for (items) |value| {
+    sum += value;
+} else 8; // result = 8
+
+// In this case, the "else" keyword is followed by a value, too.
+// However, the syntax is different: it is labeled.
+// Instead of a value, there is a label followed by a block of code, which
+// allows to do stuff before returning the value (see the "break" invocation).
+const result = for (items) |value| { // First: loop.
+    sum += value;
+} else blk: {                        // Second: "else" block.
+    std.log.info("executed AFTER the loop!", .{});
+    break :blk sum; // The "sum" value will replace the label "blk".
+};
+```
+
+### Switch.
+```zig
+
+// As a switch in C, but slightly more advanced.
+// Syntax:
+//   switch (value) {
+//       pattern => expression,
+//       pattern => expression,
+//       else    => expression
+//   };
+
+// A switch only checking for simple values.
+var x = switch(value) {
+    Error.UnExpected     => return err,
+    Error.Authentication => unreachable,
+    else                 => unreachable,
+};
+
+// A slightly more advanced switch, accepting a range of values:
+const foo: i32 = 0;
+const bar = switch (foo) {
+  0                        => "zero",
+  1...std.math.maxInt(i32) => "positive",
+  else                     => "negative",
+};
+```
+
+### Structures.
+```zig
+
+// Structure containing a single value.
+const Full = struct {
+    number: u16,
+};
+
+// Packed structure, with guaranteed in-memory layout.
+const Divided = packed struct {
+    half1: u8,
+    quarter3: u4,
+    quarter4: u4,
+};
+
+// Point is a constant representing a structure containing two u32, "x" and "y".
+// "x" has a default value, which wasn't possible in C.
+const Point = struct {
+    x: u32 = 1, // default value
+    y: u32,
+};
+
+// Variable "p" is a new Point, with x = 1 (default value) and y = 2.
+var p = Point{ .y = 2 };
+
+// Fields are accessed as usual with the dot notation: variable.field.
+print("p.x: {}\n", .{p.x}); // 1
+print("p.y: {}\n", .{p.y}); // 2
+
+
+// A structure can also contain public constants and functions.
+const Point = struct {
+    pub const some_constant = 30;
+
+    x: u32,
+    y: u32,
+
+    // This function "init" creates a Point and returns it.
+    pub fn init() Point {
+        return Point{ .x = 0, .y = 0 };
+    }
+};
+
+
+// How to access a structure public constant.
+// The value isn't accessed from an "instance" of the structure, but from the
+// constant representing the structure definition (Point).
+print("constant: {}\n", .{Point.some_constant});
+
+// Having an "init" function is rather idiomatic in the standard library.
+// More on that later.
+var p = Point.init();
+print("p.x: {}\n", .{p.x}); // p.x = 0
+print("p.y: {}\n", .{p.y}); // p.y = 0
+
+
+// Structures often have functions to modify their state, similar to
+// object-oriented programming.
+const Point = struct {
+    const Self = @This(); // Refers to its own type (later called "Point").
+
+    x: u32,
+    y: u32,
+
+    // Take a look at the signature. First argument is of type *Self: "self" is
+    // a pointer on the instance of the structure.
+    // This allows the same "dot" notation as in OOP, like "instance.set(x,y)".
+    // See the following example.
+    pub fn set(self: *Self, x: u32, y: u32) void {
+        self.x = x;
+        self.y = y;
+    }
+
+    // Again, look at the signature. First argument is of type Self (not *Self),
+    // this isn't a pointer. In this case, "self" refers to the instance of the
+    // structure, but can't be modified.
+    pub fn getx(self: Self) u32 {
+        return self.x;
+    }
+
+    // PS: two previous functions may be somewhat useless.
+    //     Attributes can be changed directly, no need for accessor functions.
+    //     It was just an example.
+};
+
+// Let's use the previous structure.
+var p = Point{ .x = 0, .y = 0 }; // "p" variable is a Point.
+
+p.set(10, 30); // x and y attributes of "p" are modified via the "set" function.
+print("p.x: {}\n", .{p.x}); // 10
+print("p.y: {}\n", .{p.y}); // 30
+
+// In C:
+//   1. We would have written something like: point_set(p, 10, 30).
+//   2. Since all functions are in the same namespace, it would have been
+//      very cumbersome to create functions with different names for different
+//      structures. Many long names, painful to read.
+//
+// In Zig, structures provide namespaces for their own functions.
+// Different structures can have the same names for their functions,
+// which brings clarity.
+```
+
+### Tuples.
+```zig
+// A tuple is a list of elements, possibly of different types.
+
+const foo = .{ "hello", true, 42 };
+// foo.len == 3
+```
+
+### Enumerations.
+```zig
+
+const Type = enum { ok, not_ok };
+
+const CardinalDirections = enum { North, South, East, West };
+const direction: CardinalDirections = .North;
+const x = switch (direction) {
+  // shorthand for CardinalDirections.North
+  .North => true,
+  else => false
+};
+
+// Switch statements need exhaustiveness.
+// WARNING: won't compile. East and West are missing.
+const x = switch (direction) {
+  .North => true,
+  .South => true,
+};
+
+
+// Switch statements need exhaustiveness.
+// Won't compile: East and West are missing.
+const x = switch (direction) {
+  .North => true,
+  .South => true,
+  .East,          // Its value is the same as the following pattern: false.
+  .West => false,
+};
+
+
+// Enumerations are like structures: they can have functions.
+```
+
+### Unions.
+```zig
+
+const Bar = union {
+  boolean: bool,
+  int: i16,
+  float: f32,
+};
+
+// Both syntaxes are equivalent.
+const foo = Bar{ .int = 42 };
+const foo: Bar = .{ .int = 42 };
+
+// Unions, like enumerations and structures, can have functions.
+```
+
+### Tagged unions.
+```zig
+
+// Unions can be declared with an enum tag type, allowing them to be used in
+// switch expressions.
+
+const MaybeEnum = enum {
+    success,
+    failure,
+};
+
+const Maybe = union(MaybeEnum) {
+    success: u8,
+    failure: []const u8,
+};
+
+// First value: success!
+const yay = Maybe{ .success = 42 };
+switch (yay) {
+    .success => |value|     std.log.info("success: {}", .{value}),
+    .failure => |err_msg|   std.log.info("failure: {}", .{err_msg}),
+}
+
+// Second value: failure! :(
+const nay = Maybe{ .failure = "I was too lazy" };
+switch (nay) {
+    .success => |value|     std.log.info("success: {}", .{value}),
+    .failure => |err_msg|   std.log.info("failure: {}", .{err_msg}),
+}
+```
+
+### Defer and errdefer.
+```zig
+
+// Make sure that an action (single instruction or block of code) is executed
+// before the end of the scope (function, block of code).
+// Even on error, that action will be executed.
+// Useful for memory allocations, and resource management in general.
+
+pub fn main() void {
+    // Should be executed at the end of the function.
+    defer print("third!\n", .{});
+
+    {
+        // Last element of its scope: will be executed right away.
+        defer print("first!\n", .{});
+    }
+
+    print("second!\n", .{});
+}
+
+fn hello_world() void {
+    defer print("end of function\n", .{}); // after "hello world!"
+
+    print("hello world!\n", .{});
+}
+
+// errdefer executes the instruction (or block of code) only on error.
+fn second_hello_world() !void {
+    errdefer print("2. something went wrong!\n", .{}); // if "foo" fails.
+    defer    print("1. second hello world\n", .{});    // executed after "foo"
+
+    try foo();
+}
+// Defer statements can be seen as stacked: first one is executed last.
+```
+
+### Memory allocators.
+Memory isn't managed directly in the standard library, instead an "allocator" is asked every time an operation on memory is required.
+Thus, the standard library lets developers handle memory as they need, through structures called "allocators", handling all memory operations.
+
+**NOTE**: the choice of the allocator isn't in the scope of this document.
+A whole book could be written about it.
+However, here are some examples, to get an idea of what you can expect:
+- page_allocator.
+  Allocate a whole page of memory each time we ask for some memory.
+  Very simple, very dumb, very wasteful.
+- GeneralPurposeAllocator.
+  Get some memory first and manage some buckets of memory in order to
+  reduce the number of allocations.
+  A bit complex. Can be combined with other allocators.
+  Can detect leaks and provide useful information to find them.
+- FixedBufferAllocator.
+  Use a fixed buffer to get its memory, don't ask memory to the kernel.
+  Very simple, limited and wasteful (can't deallocate), but very fast.
+- ArenaAllocator.
+  Allow to free all allocted memory at once.
+  To use in combinaison with another allocator.
+  Very simple way of avoiding leaks.
+
+A first example.
+```zig
+// "!void" means the function doesn't return any value except for errors.
+// In this case we try to allocate memory, and this may fail.
+fn foo() !void {
+    // In this example we use a page allocator.
+    var allocator = std.heap.page_allocator;
+
+    // "list" is an ArrayList of 8-bit unsigned integers.
+    // An ArrayList is a contiguous, growable list of elements in memory.
+    var list = try ArrayList(u8).initAllocated(allocator);
+    defer list.deinit(); // Free the memory at the end of the scope. Can't leak.
+    // "defer" allows to express memory release right after its allocation,
+    // regardless of the complexity of the function (loops, conditions, etc.).
+
+    list.add(5); // Some memory is allocated here, with the provided allocator.
+
+    for (list.items) |item| {
+        std.debug.print("item: {}\n", .{item});
+    }
+}
+```
+
+### Memory allocation combined with error management and defer.
+```zig
+
+fn some_memory_allocation_example() !void {
+    // Memory allocation may fail, so we "try" to allocate the memory and
+    // in case there is an error, the current function returns it.
+    var buf = try page_allocator.alloc(u8, 10);
+    // Defer memory release right after the allocation.
+    // Will happen even if an error occurs.
+    defer page_allocator.free(buf);
+
+    // Second allocation.
+    // In case of a failure, the first allocation is correctly released.
+    var buf2 = try page_allocator.alloc(u8, 10);
+    defer page_allocator.free(buf2);
+
+    // In case of failure, both previous allocations are correctly deallocated.
+    try foo();
+    try bar();
+
+    // ...
+}
+```
+
+### Memory allocators: a taste of the standard library.
+```zig
+
+// Allocators: 4 main functions to know
+//   single_value = create (type)
+//   destroy (single_value)
+//   slice = alloc (type, size)
+//   free (slice)
+
+// Page Allocator
+fn page_allocator_fn() !void {
+    var slice = try std.heap.page_allocator.alloc(u8, 3);
+    defer std.heap.page_allocator.free(slice);
+
+    // playing_with_a_slice(slice);
+}
+
+// GeneralPurposeAllocator
+fn general_purpose_allocator_fn() !void {
+    // GeneralPurposeAllocator has to be configured.
+    // In this case, we want to track down memory leaks.
+    const config = .{.safety = true};
+    var gpa = std.heap.GeneralPurposeAllocator(config){};
+    defer _ = gpa.deinit();
+
+    const allocator = gpa.allocator();
+
+    var slice = try allocator.alloc(u8, 3);
+    defer allocator.free(slice);
+
+    // playing_with_a_slice(slice);
+}
+
+// FixedBufferAllocator
+fn fixed_buffer_allocator_fn() !void {
+    var buffer = [_]u8{0} ** 1000; // array of 1000 u8, all initialized at zero.
+    var fba  = std.heap.FixedBufferAllocator.init(buffer[0..]);
+    // Side note: buffer[0..] is a way to create a slice from an array.
+    //            Since the function takes a slice and not an array, this makes
+    //            the type system happy.
+
+    var allocator = fba.allocator();
+
+    var slice = try allocator.alloc(u8, 3);
+    // No need for "free", memory cannot be freed with a fixed buffer allocator.
+    // defer allocator.free(slice);
+
+    // playing_with_a_slice(slice);
+}
+
+// ArenaAllocator
+fn arena_allocator_fn() !void {
+    // Reminder: arena doesn't allocate memory, it uses an inner allocator.
+    // In this case, we combine the arena allocator with the page allocator.
+    var arena = std.heap.arena_allocator.init(std.heap.page_allocator);
+    defer arena.deinit(); // end of function = all allocations are freed.
+
+    var allocator = arena.allocator();
+
+    const slice = try allocator.alloc(u8, 3);
+    // No need for "free", memory will be freed anyway.
+
+    // playing_with_a_slice(slice);
+}
+
+
+// Combining the general purpose and arena allocators. Both are very useful,
+// and their combinaison should be in everyone's favorite cookbook.
+fn gpa_arena_allocator_fn() !void {
+    const config = .{.safety = true};
+    var gpa = std.heap.GeneralPurposeAllocator(config){};
+    defer _ = gpa.deinit();
+
+    const gpa_allocator = gpa.allocator();
+
+    var arena = arena_allocator.init(gpa_allocator);
+    defer arena.deinit();
+
+    const allocator = arena.allocator();
+
+    var slice = try allocator.alloc(u8, 3);
+    defer allocator.free(slice);
+
+    // playing_with_a_slice(slice);
+}
+```
+
+### Comptime.
+```zig
+
+// Comptime is a way to avoid the pre-processor.
+// The idea is simple: run code at compilation.
+
+inline fn max(comptime T: type, a: T, b: T) T {
+    return if (a > b) a else b;
+}
+
+var res = max(u64, 1, 2);
+var res = max(f32, 10.50, 32.19);
+
+
+// Comptime: creating generic structures.
+
+fn List(comptime T: type) type {
+    return struct {
+        items: []T,
+
+        fn init()   ... { ... }
+        fn deinit() ... { ... }
+        fn do()     ... { ... }
+    };
+}
+
+const MyList = List(u8);
+
+
+// use
+var list = MyList{
+    .items = ... // memory allocation
+};
+
+list.items[0] = 10;
+```
+
+### Conditional compilation.
+```zig
+const available_os = enum { OpenBSD, Linux };
+const myos = available_os.OpenBSD;
+
+
+// The following switch is based on a constant value.
+// This means that the only possible outcome is known at compile-time.
+// Thus, there is no need to build the rest of the possibilities.
+// Similar to the "#ifdef" in C, but without requiring a pre-processor.
+const string = switch (myos) {
+   .OpenBSD => "OpenBSD is awesome!",
+   .Linux => "Linux rocks!",
+};
+
+// Also works in this case.
+const myprint = switch(myos) {
+    .OpenBSD => std.debug.print,
+    .Linux => std.log.info,
+}
+```
+
+### Testing our functions.
+```zig
+const std = @import("std");
+const expect = std.testing.expect;
+
+// Function to test.
+pub fn some_function() bool {
+    return true;
+}
+
+// This "test" block can be run with "zig test".
+// It will test the function at compile-time.
+test "returns true" {
+    expect(false == some_function());
+}
+```
+
+### Compiler built-ins.
+The compiler has special functions called "built-ins", starting with an "@".
+There are more than a hundred built-ins, allowing very low-level stuff:
+- compile-time errors, logging, verifications
+- type coercion and convertion, even in an unsafe way
+- alignment management
+- memory tricks (such as getting the byte offset of a field in a struct)
+- calling functions at compile-time
+- including C headers to transparently call C functions
+- atomic operations
+- embed files into the executable (@embedFile)
+- frame manipulations (for async functions, for example)
+- etc.
+
+Example: enums aren't integers, they have to be converted with a built-in.
+```zig
+const Value = enum { zero, stuff, blah };
+if (@enumToInt(Value.zero)  == 0) { ... }
+if (@enumToInt(Value.stuff) == 1) { ... }
+if (@enumToInt(Value.blah)  == 2) { ... }
+```
+
+
+### A few "not yourself in the foot" measures in the Zig language.
+
+- Namespaces: names conflicts are easily avoided.
+  In practice, that means an unified API between different structures (data types).
+- Enumerations aren't integers. Comparing an enumeration to an integer requires a conversion.
+- Explicit casts, coercion exists but is limited.
+  Types are slightly more enforced than in C, just a taste:
+    Pointers aren't integers, explicit conversion is necessary.
+    You won't lose precision by accident, implicit coercions are only authorized in case no precision can be lost.
+    Unions cannot be reinterpreted (in an union with an integer and a float, one cannot take a value for another by accident).
+    Etc.
+- Removing most of the C undefined behaviors (UBs), and when the compiler encounters one, it stops.
+- Slice and Array structures are prefered to pointers.
+  Types enforced by the compiler are less prone to errors than pointer manipulations.
+- Numerical overflows produce an error, unless explicitly accepted using wrapping operators.
+- Try and catch mechanism.
+  It's both handy, trivially implemented (simple error enumeration), and it takes almost no space nor computation time.
+- Unused variables are considered as errors by the compiler.
+- Many pointer types exist in order to represent what is pointed.
+  Example: is this a single value or an array, is the length known, etc.
+- Structures need a value for their attributes, and it still is possible to give an undefined value (stack garbage), but at least it is explicitely undefined.
+
+
+## Further Reading
+
+For a start, some concepts are presented on the [Zig learn website][ziglearn].
+
+The [official website][zigdoc] provides a reference documentation to the language.
+
+For now, documentation for standard library is WIP.
+
+[ziglang]: https://ziglang.org
+[ziglearn]: https://ziglearn.org/
+[zigdoc]: https://ziglang.org/documentation/
author	Karchnu <karchnu@karchnu.fr>	2021-12-25 23:42:15 +0100
committer	Karchnu <karchnu@karchnu.fr>	2021-12-25 23:42:15 +0100
commit	077c01cdf358e8d832c0ac720744e061e276c815 (patch)
tree	383544f7835517720d7a953a3f2e93c9df116005 /zig.html.markdown
parent	fd3b5cf62961c15f59acb63dad86865fc01ecb78 (diff)