| Author: | Andreas Rumpf |
|---|---|
| Version: | |nimversion| |
Contents
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. If you would like to have a gentle introduction of those concepts, we recommend Nim Basics tutorial. On the other hand, 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.
We start the tour with a modified "hello world" program:
Save this code to the file "greetings.nim". Now compile and run it:
nim compile --run greetings.nim
With the --run switch Nim
executes the file automatically after compilation. You can give your program
command-line arguments by appending them after the filename:
nim compile --run greetings.nim arg1 arg2
Commonly used commands and switches have abbreviations, so you can also use:
nim c -r greetings.nim
To compile a release version use:
nim c -d:release greetings.nim
By default, the Nim compiler generates a large number of runtime checks
aiming for your debugging pleasure. With -d:release some checks are
turned off and optimizations are turned on.
Though it should be pretty obvious what the program does, I will explain the syntax: statements which are not indented are executed when the program starts. Indentation is Nim's way of grouping statements. Indentation is done with spaces only, tabulators are not allowed.
String literals are enclosed in double-quotes. The var statement declares
a new variable named name of type string with the value that is
returned by the readLine procedure. Since the
compiler knows that readLine returns a string,
you can leave out the type in the declaration (this is called `local type
inference`:idx:). So this will work too:
Note that this is basically the only form of type inference that exists in Nim: it is a good compromise between brevity and readability.
The "hello world" program contains several identifiers that are already known
to the compiler: echo, readLine, etc.
These built-ins are declared in the system module which is implicitly
imported by any other module.
Let us look at Nim's lexical elements in more detail: like other programming languages Nim consists of (string) literals, identifiers, keywords, comments, operators, and other punctuation marks.
String literals are enclosed in double-quotes; character literals in single
quotes. Special characters are escaped with \\: \n means newline, \t
means tabulator, etc. There are also raw string literals:
In raw literals, the backslash is not an escape character.
The third and last way to write string literals is long-string literals.
They are written with three quotes: """ ... """; they can span over
multiple lines and the \\ is not an escape character either. They are very
useful for embedding HTML code templates for example.
Comments start anywhere outside a string or character literal with the
hash character #. Documentation comments start with ##:
Documentation comments are tokens; they are only allowed at certain places in the input file as they belong to the syntax tree! This feature enables simpler documentation generators.
Multiline comments are started with #[ and terminated with ]#. Multiline
comments can also be nested.
Numerical literals are written as in most other languages. As a special twist,
underscores are allowed for better readability: 1_000_000 (one million).
A number that contains a dot (or 'e' or 'E') is a floating-point literal:
1.0e9 (one billion). Hexadecimal literals are prefixed with 0x,
binary literals with 0b and octal literals with 0o. A leading zero
alone does not produce an octal.
The var statement declares a new local or global variable:
Indentation can be used after the var keyword to list a whole section of
variables:
The assignment statement assigns a new value to a variable or more generally to a storage location:
= is the assignment operator. The assignment operator can be
overloaded. You can declare multiple variables with a single assignment
statement and all the variables will have the same value:
Note that declaring multiple variables with a single assignment that calls a procedure can have unexpected results: the compiler will unroll the assignments and end up calling the procedure several times. If the result of the procedure depends on side effects, your variables may end up having different values! For safety use side-effect-free procedures if making multiple assignments.
Constants are symbols which are bound to a value. The constant's value cannot change. The compiler must be able to evaluate the expression in a constant declaration at compile time:
Indentation can be used after the const keyword to list a whole section of
constants:
The let statement works like the var statement but the declared
symbols are single assignment variables: After the initialization their
value cannot change:
The difference between let and const is: let introduces a variable
that can not be re-assigned, const means "enforce compile time evaluation
and put it into a data section":
The greetings program consists of 3 statements that are executed sequentially. Only the most primitive programs can get away with that: branching and looping are needed too.
The if statement is one way to branch the control flow:
There can be zero or more elif parts, and the else part is optional.
The keyword elif is short for else if, and is useful to avoid
excessive indentation. (The "" is the empty string. It contains no
characters.)
Another way to branch is provided by the case statement. A case statement is a multi-branch:
As it can be seen, for an of branch a comma-separated list of values is also
allowed.
The case statement can deal with integers, other ordinal types, and strings. (What an ordinal type is will be explained soon.) For integers or other ordinal types value ranges are also possible:
However, the above code does not compile: the reason is that you have to cover
every value that n may contain, but the code only handles the values
0..8. Since it is not very practical to list every other possible integer
(though it is possible thanks to the range notation), we fix this by telling
the compiler that for every other value nothing should be done:
The empty discard statement is a do
nothing statement. The compiler knows that a case statement with an else part
cannot fail and thus the error disappears. Note that it is impossible to cover
all possible string values: that is why string cases always need an else
branch.
In general, the case statement is used for subrange types or enumerations where it is of great help that the compiler checks that you covered any possible value.
The while statement is a simple looping construct:
The example uses a while loop to keep asking the users for their name, as long as the user types in nothing (only presses RETURN).
The for statement is a construct to loop over any element an iterator
provides. The example uses the built-in countup iterator:
The variable i is implicitly declared by the
for loop and has the type int, because that is what countup returns. i runs through the values
1, 2, .., 10. Each value is echo-ed. This code does the same:
Counting down can be achieved as easily (but is less often needed):
Since counting up occurs so often in programs, Nim also has a .. iterator that does the same:
Zero-indexed counting has two shortcuts ..< and .. ^1
(backward index operator) to simplify
counting to one less than the higher index:
or
or
Other useful iterators for collections (like arrays and sequences) are
* items and mitems, which provides immutable and mutable elements respectively, and
* pairs and mpairs which provides the element and an index number (immutable and mutable respectively)
Control flow statements have a feature not covered yet: they open a
new scope. This means that in the following example, x is not accessible
outside the loop:
A while (for) statement introduces an implicit block. Identifiers
are only visible within the block they have been declared. The block
statement can be used to open a new block explicitly:
The block's label (myblock in the example) is optional.
A block can be left prematurely with a break statement. The break statement
can leave a while, for, or a block statement. It leaves the
innermost construct, unless a label of a block is given:
Like in many other programming languages, a continue statement starts
the next iteration immediately:
Example:
The when statement is almost identical to the if statement, but with these
differences:
- Each condition must be a constant expression since it is evaluated by the compiler.
- The statements within a branch do not open a new scope.
- The compiler checks the semantics and produces code only for the statements
that belong to the first condition that evaluates to
true.
The when statement is useful for writing platform-specific code, similar to
the #ifdef construct in the C programming language.
Now that we covered the basic control flow statements, let's return to Nim indentation rules.
In Nim, there is a distinction between simple statements and complex
statements. Simple statements cannot contain other statements:
Assignment, procedure calls, or the return statement are all simple
statements. Complex statements like if, when, for, while can
contain other statements. To avoid ambiguities, complex statements must always
be indented, but single simple statements do not:
Expressions are parts of a statement that usually result in a value. The condition in an if statement is an example of an expression. Expressions can contain indentation at certain places for better readability:
if thisIsaLongCondition() and
thisIsAnotherLongCondition(1,
2, 3, 4):
x = trueAs a rule of thumb, indentation within expressions is allowed after operators, an open parenthesis and after commas.
With parenthesis and semicolons (;) you can use statements where only
an expression is allowed:
To define new commands like echo
and readLine in the examples, the concept of a
procedure is needed. (Some languages call them methods or functions.)
In Nim new procedures are defined with the proc keyword:
This example shows a procedure named yes that asks the user a question
and returns true if they answered "yes" (or something similar) and returns
false if they answered "no" (or something similar). A return statement
leaves the procedure (and therefore the while loop) immediately. The
(question: string): bool syntax describes that the procedure expects a
parameter named question of type string and returns a value of type
bool. The bool type is built-in: the only valid values for bool are
true and false.
The conditions in if or while statements must be of type bool.
Some terminology: in the example question is called a (formal) parameter,
"Should I..." is called an argument that is passed to this parameter.
A procedure that returns a value has an implicit result variable declared
that represents the return value. A return statement with no expression is
shorthand for return result. The result value is always returned
automatically at the end of a procedure if there is no return statement at
the exit.
The result variable is already implicitly declared at the start of the
function, so declaring it again with 'var result', for example, would shadow it
with a normal variable of the same name. The result variable is also already
initialized with the type's default value. Note that referential data types will
be nil at the start of the procedure, and thus may require manual
initialization.
A procedure that does not have any return statement and does not use the
special result variable returns the value of its last expression. For example,
this procedure
returns the string "Hello, World!".
Parameters are immutable in the procedure body. By default, their value cannot be
changed because this allows the compiler to implement parameter passing in the
most efficient way. If a mutable variable is needed inside the procedure, it has
to be declared with var in the procedure body. Shadowing the parameter name
is possible, and actually an idiom:
If the procedure needs to modify the argument for the
caller, a var parameter can be used:
In the example, res and remainder are var parameters.
Var parameters can be modified by the procedure and the changes are
visible to the caller. Note that the above example would better make use of
a tuple as a return value instead of using var parameters.
To call a procedure that returns a value just for its side effects and ignoring
its return value, a discard statement must be used. Nim does not
allow silently throwing away a return value:
The return value can be ignored implicitly if the called proc/iterator has
been declared with the discardable pragma:
Often a procedure has many parameters and it is not clear in which order the parameters appear. This is especially true for procedures that construct a complex data type. Therefore the arguments to a procedure can be named, so that it is clear which argument belongs to which parameter:
Now that we use named arguments to call createWindow the argument order
does not matter anymore. Mixing named arguments with ordered arguments is
also possible, but not very readable:
The compiler checks that each parameter receives exactly one argument.
To make the createWindow proc easier to use it should provide default
values; these are values that are used as arguments if the caller does not
specify them:
Now the call to createWindow only needs to set the values that differ
from the defaults.
Note that type inference works for parameters with default values; there is
no need to write title: string = "unknown", for example.
Nim provides the ability to overload procedures similar to C++:
(Note that toString is usually the $ operator in
Nim.) The compiler chooses the most appropriate proc for the toString
calls. How this overloading resolution algorithm works exactly is not
discussed here (it will be specified in the manual soon). However, it does
not lead to nasty surprises and is based on a quite simple unification
algorithm. Ambiguous calls are reported as errors.
The Nim library makes heavy use of overloading - one reason for this is that
each operator like + is just an overloaded proc. The parser lets you
use operators in infix notation (a + b) or prefix notation (+ a).
An infix operator always receives two arguments, a prefix operator always one.
(Postfix operators are not possible, because this would be ambiguous: does
a @ @ b mean (a) @ (@b) or (a@) @ (b)? It always means
(a) @ (@b), because there are no postfix operators in Nim.)
Apart from a few built-in keyword operators such as and, or, not,
operators always consist of these characters:
+ - * \ / < > = @ $ ~ & % ! ? ^ . |
User-defined operators are allowed. Nothing stops you from defining your own
@!?+~ operator, but doing so may reduce readability.
The operator's precedence is determined by its first character. The details can be found in the manual.
To define a new operator enclose the operator in backticks "`":
The "`" notation can also be used to call an operator just like any other procedure:
Every variable, procedure, etc. needs to be declared before it can be used. (The reason for this is that it is non-trivial to avoid this need in a language that supports metaprogramming as extensively as Nim does.) However, this cannot be done for mutually recursive procedures:
Here odd depends on even and vice versa. Thus even needs to be
introduced to the compiler before it is completely defined. The syntax for
such a forward declaration is simple: just omit the = and the
procedure's body. The assert just adds border conditions, and will be
covered later in Modules section.
Later versions of the language will weaken the requirements for forward declarations.
The example also shows that a proc's body can consist of a single expression whose value is then returned implicitly.
Let's return to the simple counting example:
Can a countup proc be written that supports this loop? Lets try:
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:
Iterators look very similar to procedures, but there are several important differences:
- Iterators can only be called from for loops.
- Iterators cannot contain a
returnstatement (and procs cannot contain ayieldstatement). - Iterators have no implicit
resultvariable. - Iterators do not support recursion.
- Iterators cannot be forward declared, because the compiler must be able to inline an iterator. (This restriction will be gone in a future version of the compiler.)
However, you can also use a closure iterator to get a different set of
restrictions. See first-class iterators
for details. Iterators can have the same name and parameters as a proc since
essentially they have their own namespaces. Therefore it is common practice to
wrap iterators in procs of the same name which accumulate the result of the
iterator and return it as a sequence, like split from the strutils module.
This section deals with the basic built-in types and the operations that are available for them in detail.
Nim's boolean type is called bool and consists of the two
pre-defined values true and false. Conditions in while,
if, elif, and when statements must be of type bool.
The operators not, and, or, xor, <, <=, >, >=, !=, == are defined
for the bool type. The and and or operators perform short-circuit
evaluation. For example:
while p != nil and p.name != "xyz":
# p.name is not evaluated if p == nil
p = p.nextThe character type is called char. Its size is always one byte, so
it cannot represent most UTF-8 characters, but it can represent one of the bytes
that makes up a multi-byte UTF-8 character.
The reason for this is efficiency: for the overwhelming majority of use-cases,
the resulting programs will still handle UTF-8 properly as UTF-8 was especially
designed for this.
Character literals are enclosed in single quotes.
Chars can be compared with the ==, <, <=, >, >= operators.
The $ operator converts a char to a string. Chars cannot be mixed
with integers; to get the ordinal value of a char use the ord proc.
Converting from an integer to a char is done with the chr proc.
String variables are mutable, so appending to a string
is possible, and quite efficient. Strings in Nim are both zero-terminated and have a
length field. A string's length can be retrieved with the builtin len
procedure; the length never counts the terminating zero. Accessing the
terminating zero is an error, it only exists so that a Nim string can be converted
to a cstring without doing a copy.
The assignment operator for strings copies the string. You can use the &
operator to concatenate strings and add to append to a string.
Strings are compared using their lexicographical order. All the comparison operators
are supported. By convention, all strings are UTF-8 encoded, but this is not
enforced. For example, when reading strings from binary files, they are merely
a sequence of bytes. The index operation s[i] means the i-th char of
s, not the i-th unichar.
A string variable is initialized with the empty string "".
Nim has these integer types built-in:
int int8 int16 int32 int64 uint uint8 uint16 uint32 uint64.
The default integer type is int. Integer literals can have a type suffix
to specify a non-default integer type:
Most often integers are used for counting objects that reside in memory, so
int has the same size as a pointer.
The common operators + - * div mod < <= == != > >= are defined for
integers. The and or xor not operators are also defined for integers and
provide bitwise operations. Left bit shifting is done with the shl, right
shifting with the shr operator. Bit shifting operators always treat their
arguments as unsigned. For `arithmetic bit shifts`:idx: ordinary
multiplication or division can be used.
Unsigned operations all wrap around; they cannot lead to over- or under-flow errors.
Lossless `Automatic type conversion`:idx: is performed in expressions where different kinds of integer types are used. However, if the type conversion would cause loss of information, the `EOutOfRange`:idx: exception is raised (if the error cannot be detected at compile time).
Nim has these floating-point types built-in: float float32 float64.
The default float type is float. In the current implementation,
float is always 64-bits.
Float literals can have a type suffix to specify a non-default float type:
The common operators + - * / < <= == != > >= are defined for
floats and follow the IEEE-754 standard.
Automatic type conversion in expressions with different kinds of floating-point types is performed: the smaller type is converted to the larger. Integer types are not converted to floating-point types automatically, nor vice versa. Use the toInt and toFloat procs for these conversions.
Conversion between numerical types is performed by using the type as a function:
As mentioned earlier, the built-in $ (stringify) operator
turns any basic type into a string, which you can then print to the console
using the echo proc. However, advanced types, and your own custom types,
won't work with the $ operator until you define it for them.
Sometimes you just want to debug the current value of a complex type without
having to write its $ operator. You can use then the repr proc which works with any type and even complex data
graphs with cycles. The following example shows that even for basic types
there is a difference between the $ and repr outputs:
In Nim new types can be defined within a type statement:
Enumeration and object types may only be defined within a
type statement.
A variable of an enumeration type can only be assigned one of the enumeration's specified values. These values are a set of ordered symbols. Each symbol is mapped to an integer value internally. The first symbol is represented at runtime by 0, the second by 1, and so on. For example:
All the comparison operators can be used with enumeration types.
An enumeration's symbol can be qualified to avoid ambiguities:
Direction.south.
The $ operator can convert any enumeration value to its name, and the ord
proc can convert it to its underlying integer value.
For better interfacing to other programming languages, the symbols of enum types can be assigned an explicit ordinal value. However, the ordinal values must be in ascending order.
Enumerations, integer types, char and bool (and
subranges) are called ordinal types. Ordinal types have quite
a few special operations:
----------------- --------------------------------------------------------
Operation Comment
----------------- --------------------------------------------------------
ord(x) returns the integer value that is used to
represent x's value
inc(x) increments x by one
inc(x, n) increments x by n; n is an integer
dec(x) decrements x by one
dec(x, n) decrements x by n; n is an integer
succ(x) returns the successor of x
succ(x, n) returns the n'th successor of x
pred(x) returns the predecessor of x
pred(x, n) returns the n'th predecessor of x
----------------- --------------------------------------------------------
The inc, dec, succ and pred operations can
fail by raising an EOutOfRange or EOverflow exception. (If the code has been
compiled with the proper runtime checks turned on.)
A subrange type is a range of values from an integer or enumeration type (the base type). Example:
MySubrange is a subrange of int which can only hold the values 0
to 5. Assigning any other value to a variable of type MySubrange is a
compile-time or runtime error. Assignments from the base type to one of its
subrange types (and vice versa) are allowed.
The system module defines the important Natural
type as range[0..high(int)] (high returns
the maximal value). Other programming languages may suggest the use of unsigned
integers for natural numbers. This is often unwise: you don't want unsigned
arithmetic (which wraps around) just because the numbers cannot be negative.
Nim's Natural type helps to avoid this common programming error.
An array is a simple fixed-length container. Each element in an array has the same type. The array's index type can be any ordinal type.
Arrays can be constructed using []:
The notation x[i] is used to access the i-th element of x.
Array access is always bounds checked (at compile-time or at runtime). These
checks can be disabled via pragmas or invoking the compiler with the
--bound_checks:off command line switch.
Arrays are value types, like any other Nim type. The assignment operator copies the whole array contents.
The built-in len proc returns the array's
length. low(a) returns the lowest valid index
for the array a and high(a) the highest
valid index.
The syntax for nested arrays (multidimensional) in other languages is a matter of appending more brackets because usually each dimension is restricted to the same index type as the others. In Nim you can have different dimensions with different index types, so the nesting syntax is slightly different. Building on the previous example where a level is defined as an array of enums indexed by yet another enum, we can add the following lines to add a light tower type subdivided into height levels accessed through their integer index:
Note how the built-in len proc returns only the array's first dimension
length. Another way of defining the LightTower to better illustrate its
nested nature would be to omit the previous definition of the LevelSetting
type and instead write it embedded directly as the type of the first dimension:
It is quite common to have arrays start at zero, so there's a shortcut syntax to specify a range from zero to the specified index minus one:
Sequences are similar to arrays but of dynamic length which may change during runtime (like strings). Since sequences are resizable they are always allocated on the heap and garbage collected.
Sequences are always indexed with an int starting at position 0. The len, low and high operations are available for sequences too.
The notation x[i] can be used to access the i-th element of x.
Sequences can be constructed by the array constructor [] in conjunction
with the array to sequence operator @. Another way to allocate space for
a sequence is to call the built-in newSeq procedure.
A sequence may be passed to an openarray parameter.
Example:
Sequence variables are initialized with @[].
The for statement can be used with one or two variables when used with a
sequence. When you use the one variable form, the variable will hold the value
provided by the sequence. The for statement is looping over the results
from the items() iterator from the system module. But if you use the two-variable form, the first
variable will hold the index position and the second variable will hold the
value. Here the for statement is looping over the results from the
pairs() iterator from the system module. Examples:
Note: Openarrays can only be used for parameters.
Often fixed-size arrays turn out to be too inflexible; procedures should be
able to deal with arrays of different sizes. The `openarray`:idx: type allows
this. Openarrays are always indexed with an int starting at position 0.
The len, low
and high operations are available for open
arrays too. Any array with a compatible base type can be passed to an
openarray parameter, the index type does not matter.
The openarray type cannot be nested: multidimensional openarrays are not supported because this is seldom needed and cannot be done efficiently.
A varargs parameter is like an openarray parameter. However, it is
also a means to implement passing a variable number of
arguments to a procedure. The compiler converts the list of arguments
to an array automatically:
This transformation is only done if the varargs parameter is the last parameter in the procedure header. It is also possible to perform type conversions in this context:
In this example $ is applied to any argument that is passed
to the parameter a. Note that $ applied to strings is a
nop.
Slices look similar to subranges types in syntax but are used in a different
context. A slice is just an object of type Slice which contains two bounds,
a and b. By itself a slice is not very useful, but other collection types
define operators which accept Slice objects to define ranges.
In the previous example slices are used to modify a part of a string. The slice's bounds can hold any value supported by their type, but it is the proc using the slice object which defines what values are accepted.
To understand some of the different ways of specifying the indices of strings, arrays, sequences, etc., it must be remembered that Nim uses zero-based indices.
So the string b is of length 19, and two different ways of specifying the
indices are
"Slices are useless."
| | |
0 11 17 using indices
^19 ^8 ^2 using ^ syntaxwhere b[0 .. ^1] is equivalent to b[0 .. b.len-1] and b[0 ..< b.len], and it
can be seen that the ^1 provides a short-hand way of specifying the b.len-1. See
the backwards index operator.
In the above example, because the string ends in a period, to get the portion of the string that is "useless" and replace it with "useful".
b[11 .. ^2] is the portion "useless", and b[11 .. ^2] = "useful" replaces the
"useless" portion with "useful", giving the result "Slices are useful."
Note 1: alternate ways of writing this are b[^8 .. ^2] = "useful" or
as b[11 .. b.len-2] = "useful" or as b[11 ..< b.len-1] = "useful".
Note 2: As the ^ template returns a distinct int
of type BackwardsIndex, we can have a lastIndex constant defined as const lastIndex = ^1,
and later used as b[0 .. lastIndex].
The default type to pack different values together in a single structure with a name is the object type. An object is a value type, which means that when an object is assigned to a new variable all its components are copied as well.
Each object type Foo has a constructor Foo(field: value, ...)
where all of its fields can be initialized. Unspecified fields will
get their default value.
Object fields that should be visible from outside the defining module have to
be marked with *.
Tuples are very much like what you have seen so far from objects. They are value types where the assignment operator copies each component. Unlike object types though, tuple types are structurally typed, meaning different tuple-types are equivalent if they specify fields of the same type and of the same name in the same order.
The constructor () can be used to construct tuples. The order of the
fields in the constructor must match the order in the tuple's
definition. But unlike objects, a name for the tuple type may not be
used here.
Like the object type the notation t.field is used to access a
tuple's field. Another notation that is not available for objects is
t[i] to access the i'th field. Here i must be a constant
integer.
Even though you don't need to declare a type for a tuple to use it, tuples created with different field names will be considered different objects despite having the same field types.
Tuples can be unpacked during variable assignment (and only then!). This can be handy to assign directly the fields of the tuples to individually named variables. An example of this is the splitFile proc from the os module which returns the directory, name, and extension of a path at the same time. For tuple unpacking to work you must use parentheses around the values you want to assign the unpacking to, otherwise, you will be assigning the same value to all the individual variables! For example:
Fields of tuples are always public, they don't need to be explicity marked to be exported, unlike for example fields in an object type.
References (similar to pointers in other programming languages) are a way to introduce many-to-one relationships. This means different references can point to and modify the same location in memory.
Nim distinguishes between `traced`:idx: and `untraced`:idx: references. Untraced references are also called pointers. Traced references point to objects in a garbage-collected heap, untraced references point to manually allocated objects or objects elsewhere in memory. Thus untraced references are unsafe. However, for certain low-level operations (e.g., accessing the hardware), untraced references are necessary.
Traced references are declared with the ref keyword; untraced references are declared with the ptr keyword.
The empty [] subscript notation can be used to de-refer a reference,
meaning to retrieve the item the reference points to. The . (access a
tuple/object field operator) and [] (array/string/sequence index operator)
operators perform implicit dereferencing operations for reference types:
To allocate a new traced object, the built-in procedure new must be used.
To deal with untraced memory, the procedures alloc, dealloc and
realloc can be used. The system
module's documentation contains further details.
If a reference points to nothing, it has the value nil.
A procedural type is a (somewhat abstract) pointer to a procedure.
nil is an allowed value for a variable of a procedural type.
Nim uses procedural types to achieve `functional`:idx: programming
techniques.
Example:
A subtle issue with procedural types is that the calling convention of the procedure influences the type compatibility: procedural types are only compatible if they have the same calling convention. The different calling conventions are listed in the manual.
A Distinct type allows for the creation of a new type that "does not imply a subtype relationship between it and its base type". You must explicitly define all behavior for the distinct type. To help with this, both the distinct type and its base type can cast from one type to the other. Examples are provided in the manual.
Nim supports splitting a program into pieces with a module concept.
Each module is in its own file. Modules enable `information hiding`:idx: and
`separate compilation`:idx:. A module may gain access to the symbols of another
module by using the `import`:idx: statement. Only top-level symbols that are marked
with an asterisk (*) are exported:
The above module exports x and *, but not y.
A module's top-level statements are executed at the start of the program. This can be used to initialize complex data structures for example.
Each module has a special magic constant isMainModule that is true if the
module is compiled as the main file. This is very useful to embed tests within
the module as shown by the above example.
A symbol of a module can be qualified with the module.symbol syntax. And if
a symbol is ambiguous, it must be qualified. A symbol is ambiguous
if it is defined in two (or more) different modules and both modules are
imported by a third one:
But this rule does not apply to procedures or iterators. Here the overloading rules apply:
The normal import statement will bring in all exported symbols.
These can be limited by naming symbols that should be excluded using
the except qualifier.
We have already seen the simple import statement that just imports all
exported symbols. An alternative that only imports listed symbols is the
from import statement:
The from statement can also force namespace qualification on
symbols, thereby making symbols available, but needing to be qualified
in order to be used.
Since module names are generally long to be descriptive, you can also define a shorter alias to use when qualifying symbols.
The include statement does something fundamentally different than
importing a module: it merely includes the contents of a file. The include
statement is useful to split up a large module into several files:
So, now that we are done with the basics, let's see what Nim offers apart from a nice syntax for procedural programming: Part II