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# Not a Nim Manual

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Authors: | Andreas Rumpf, Zahary Karadjov
Version: | |nimversion|

> "Complexity" seems to be a lot like "energy": you can transfer it from the end-user to one/some of the other players, but the total amount seems to remain pretty much constant for a given task. -- Ran

# About this document

Note: This document is a draft! Several of Nim's features may need more precise wording. This manual is constantly evolving into a proper specification.

Note: The experimental features of Nim are covered here.

Note: Assignments, moves, and destruction are specified in the destructors document.

The language constructs are explained using an extended BNF, in which `(a)*` means 0 or more `a`'s, `a+` means 1 or more `a`'s, and `(a)?` means an optional a. Parentheses may be used to group elements.

`&` is the lookahead operator; `&a` means that an `a` is expected but not consumed. It will be consumed in the following rule.

Non-terminals start with a lowercase letter, abstract terminal symbols are in UPPERCASE. Verbatim terminal symbols (including keywords) are quoted with `'`. An example:

    ifStmt = 'if' expr ':' stmts ('elif' expr ':' stmts)* ('else' stmts)?

In a typical Nim program, most of the code is compiled into the executable. However, some of the code may be executed at compile-time. This can include constant expressions, macro definitions, and Nim procedures used by macro definitions. Most of the Nim language is supported at compile-time, but there are some restrictions -- see Restrictions on Compile-Time Execution for details. We use the term runtime to cover both compile-time execution and code execution in the executable.

    var a: array[0..1, char]
    let i = 5
    try:
      a[i] = 'N'
    except IndexDefect:
      echo "invalid index"

## Encoding

All Nim source files are in the UTF-8 encoding (or its ASCII subset). Other encodings are not supported. Any of the standard platform line termination sequences can be used - the Unix form using ASCII LF (linefeed), the Windows form using the ASCII sequence CR LF (return followed by linefeed), or the old Macintosh form using the ASCII CR (return) character. All of these forms can be used equally, regardless of the platform.

## Indentation

Nim's standard grammar describes an indentation sensitive language. This means that all the control structures are recognized by indentation. Indentation consists only of spaces; tabulators are not allowed.

With this notation we can now easily define the core of the grammar: A block of statements (simplified example):

    ifStmt = 'if' expr ':' stmt
             (IND{=} 'elif' expr ':' stmt)*
             (IND{=} 'else' ':' stmt)?

    simpleStmt = ifStmt / ...

    stmt = IND{>} stmt ^+ IND{=} DED  # list of statements
         / simpleStmt                 # or a simple statement

String literals can be delimited by matching double quotes, and can contain the following escape sequences:

Escape sequence | Meaning
--- | ---
`\p` | platform specific newline: CRLF on Windows, LF on Unix
`\r`, `\c` | carriage return
`\n`, `\l` | line feed (often called newline)
`\f` | form feed
`\t` | tabulator
`\v` | vertical tabulator
`\\` | backslash
`\"` | quotation mark
`\'` | apostrophe
`\` '0'..'9'+ | character with decimal value d; all decimal digits directly following are used for the character
`\a` | alert
`\b` | backspace
`\e` | escape [ESC]
`\x` HH | character with hex value HH; exactly two hex digits are allowed
`\u` HHHH | unicode codepoint with hex value HHHH; exactly four hex digits are allowed
`\u` {H+} | unicode codepoint; all hex digits enclosed in `{}` are used for the codepoint

    """"long string within quotes""""

Produces:

    "long string within quotes"

## Operators

Nim allows user defined operators. An operator is any combination of the following characters:

    =     +     -     *     /     <     >
    @     $     ~     &     %     |
    !     ?     ^     .     :     \

(The grammar uses the terminal OPR to refer to operator symbols as defined here.)

The following strings denote other tokens:

    `   (    )     {    }     [    ]    ,  ;   [.    .]  {.   .}  (.  .)  [:

Otherwise, precedence is determined by the first character.

Precedence level | Operators | First character | Terminal symbol
--- | --- | --- | ---
10 (highest) |  | `$ ^` | OP10
9 | `* / div mod shl shr %` | `* % \ /` | OP9
8 | `+ -` | `+ - ~ |` | OP8
7 | `&` | `&` | OP7
6 | `..` | `.` | OP6
5 | `== <= < >= > != in notin is isnot not of as from` | `= < > !` | OP5
4 | `and` |  | OP4
3 | `or xor` |  | OP3
2 |  | `@ : ?` | OP2
1 | assignment operator (like `+=`, `*=`) |  | OP1
0 (lowest) | arrow like operator (like `->`, `=>`) |  | OP0

# Constants and Constant Expressions

A constant is a symbol that is bound to the value of a constant expression. Constant expressions are restricted to depend only on the following categories of values and operations, because these are either built into the language or declared and evaluated before semantic analysis of the constant expression:

- literals
- built-in operators
- previously declared constants and compile-time variables
- previously declared macros and templates
- previously declared procedures that have no side effects beyond possibly modifying compile-time variables

These integer types are pre-defined:

`int`
  the generic signed integer type; its size is platform-dependent and has the same size as a pointer. This type should be used in general. An integer literal that has no type suffix is of this type if it is in the range `low(int32)..high(int32)` otherwise the literal's type is `int64`.
intXX
  additional signed integer types of XX bits use this naming scheme (example: int16 is a 16-bit wide integer). The current implementation supports `int8`, `int16`, `int32`, `int64`. Literals of these types have the suffix 'iXX.
`uint`
  the generic unsigned integer type; its size is platform-dependent and has the same size as a pointer. An integer literal with the type suffix `'u` is of this type.

Let `T`'s be `p`'s return type. NRVO applies for `T` if `sizeof(T) >= N` (where `N` is implementation dependent), in other words, it applies for "big" structures.

Apart from built-in operations like array indexing, memory allocation, etc. the `raise` statement is the only way to raise an exception.

`typedesc` used as a parameter type also introduces an implicit generic. `typedesc` has its own set of rules:

The `!=`, `>`, `>=`, `in`, `notin`, `isnot` operators are in fact templates:

`a > b` is transformed into `b < a`.
`a in b` is transformed into `contains(b, a)`.
`notin` and `isnot` have the obvious meanings.

A template where every parameter is `untyped` is called an immediate template. For historical reasons templates can be explicitly annotated with an `immediate` pragma and then these templates do not take part in overloading resolution and the parameters' types are ignored by the compiler. Explicit immediate templates are now deprecated.

## Symbol lookup in generics

### Open and Closed symbols

The symbol binding rules in generics are slightly subtle: There are "open" and "closed" symbols. A "closed" symbol cannot be re-bound in the instantiation context, an "open" symbol can. Per default overloaded symbols are open and every other symbol is closed.

In templates identifiers can be constructed with the backticks notation:

    template typedef(name: untyped, typ: typedesc) =
      type
        `T name`* {.inject.} = typ
        `P name`* {.inject.} = ref `T name`

    typedef(myint, int)
    var x: PMyInt

In the example `name` is instantiated with `myint`, so `T name` becomes `Tmyint`.

Only top-level symbols that are marked with an asterisk (`*`) are exported.

The algorithm for compiling modules is:

- compile the whole module as usual, following import statements recursively
- if there is a cycle only import the already parsed symbols (that are exported); if an unknown identifier occurs then abort

### Collective imports from a directory

The syntax `import dir / [moduleA, moduleB]` can be used to import multiple modules from the same directory.

# Pragmas

Pragmas are Nim's method to give the compiler additional information / commands without introducing a massive number of new keywords. Pragmas are processed on the fly during semantic checking. Pragmas are enclosed in the special `{.` and `.}` curly brackets. Pragmas are also often used as a first implementation to play with a language feature before a nicer syntax to access the feature becomes available.

## deprecated pragma

The deprecated pragma is used to mark a symbol as deprecated:

Note: c2nim can parse a large subset of C++ and knows about the `importcpp` pragma pattern language. It is not necessary to know all the details described here.

Pure libraries do not depend on any external `*.dll` or `lib*.so` binary while impure libraries do. A wrapper is an impure library that is a very low-level interface to a C library.

# Pure libraries

## Automatic imports

- system Basic procs and operators that every program needs. It also provides IO facilities for reading and writing text and binary files. It is imported implicitly by the compiler. Do not import it directly. It relies on compiler magic to work.
- threads Basic Nim thread support. Note: This is part of the system module. Do not import it explicitly. Enabled with `--threads:on`.

# Code reordering

The code reordering feature can implicitly rearrange procedure, template, and macro definitions along with variable declarations and initializations at the top level scope so that, to a large extent, a programmer should not have to worry about ordering definitions correctly or be forced to use forward declarations to preface definitions inside a module.

Example:

    {.experimental: "codeReordering".}

    proc foo(x: int) =
      bar(x)

    proc bar(x: int) =
      echo(x)

    foo(10)

It is important to note that reordering only works for symbols at top level scope. Therefore, the following will fail to compile:

## Parameter constraints

The parameter constraint expression can use the operators `|` (or), `&` (and) and `~` (not) and the following predicates:

### The `~` operator

The `~` operator is the not operator in patterns:

### The `**` operator

The `**` is much like the `*` operator, except that it gathers not only all the arguments, but also the matched operators in reverse polish notation:

Nim significantly improves on the safety of these features via additional pragmas:

1. A guard annotation is introduced to prevent data races.
2. Every access of a guarded memory location needs to happen in an appropriate locks statement.
3. Locks and routines can be annotated with lock levels to allow potential deadlocks to be detected during semantic analysis.

1. Two output parameters should never be aliased.
2. An input and an output parameter should not be aliased.
3. An output parameter should never be aliased with a global or thread local variable referenced by the called proc.
4. An input parameter should not be aliased with a global or thread local variable updated by the called proc.

One problem with rules 3 and 4 is that they affect specific global or thread local variables, but Nim's effect tracking only tracks "uses no global variable" via `.noSideEffect`. The rules 3 and 4 can also be approximated by a different rule:

5. A global or thread local variable (or a location derived from such a location) can only passed to a parameter of a `.noSideEffect` proc.

These two procs are the two modus operandi of the real-time garbage collector:

1. GC_SetMaxPause Mode

   You can call `GC_SetMaxPause` at program startup and then each triggered garbage collector run tries to not take longer than `maxPause` time. However, it is possible (and common) that the work is nevertheless not evenly distributed as each call to `new` can trigger the garbage collector and thus take `maxPause` time.

2. GC_step Mode

   This allows the garbage collector to perform some work for up to `us` time. This is useful to call in the main loop to ensure the garbage collector can do its work. To bind all garbage collector activity to a `GC_step` call, deactivate the garbage collector with `GC_disable` at program startup. If `strongAdvice` is set to `true`, then the garbage collector will be forced to perform the collection cycle. Otherwise, the garbage collector may decide not to do anything, if there is not much garbage to collect. You may also specify the current stack size via `stackSize` parameter. It can improve performance when you know that there are no unique Nim references below a certain point on the stack. Make sure the size you specify is greater than the potential worst-case size.

   It can improve performance when you know that there are no unique Nim references below a certain point on the stack. Make sure the size you specify is greater than the potential worst-case size.

These procs provide a "best effort" real-time guarantee; in particular the cycle collector is not aware of deadlines. Deactivate it to get more predictable real-time behaviour. Tests show that a 1ms max pause time will be met in almost all cases on modern CPUs (with the cycle collector disabled).

## Time measurement with garbage collectors

The garbage collectors' way of measuring time uses (see `lib/system/timers.nim` for the implementation):

1. `QueryPerformanceCounter` and `QueryPerformanceFrequency` on Windows.
2. `mach_absolute_time` on Mac OS X.
3. `gettimeofday` on Posix systems.

As such it supports a resolution of nanoseconds internally; however, the API uses microseconds for convenience.

# Introduction

> "Der Mensch ist doch ein Augentier -- schöne Dinge wünsch ich mir."

This document is a tutorial for the programming language Nim. This tutorial assumes that you are familiar with basic programming concepts like variables, types, or statements but is kept very basic. The manual contains many more examples of the advanced language features. All code examples in this tutorial, as well as the ones found in the rest of Nim's documentation, follow the Nim style guide.

However, this does not work. The problem is that the procedure should not only `return`, but return and continue after an iteration has finished. This return and continue is called a `yield` statement. Now the only thing left to do is to replace the `proc` keyword by `iterator` and here it is - our first iterator:

A1 header | A2 | not fooled
--- | ---
C1 | C2 bold
D1 `code \|` | D2
E1 | text |
| F2 without pipe

not in table