This document is a tutorial for the programming language *Nimrod*. After this tutorial you will have a decent knowledge of Nimrod. This tutorial assumes that you are familiar with basic programming concepts like variables, types or statements. The first program ================= We start the tour with a modified "hello world" program: .. code-block:: Nimrod # This is a comment Echo("What's your name? ") var name: string = readLine(stdin) Echo("Hi, ", name, "!") Save this code to the file "greetings.nim". Now compile and run it:: nimrod compile --run greetings.nim With the ``--run`` switch Nimrod executes the file automatically after compilation. You can give your program command line arguments by appending them after the filename:: nimrod compile --run greetings.nim arg1 arg2 Commonly used commands and switches have abbreviations, so you can also use:: nimrod c -r greetings.nim To compile a release version use:: nimrod c -d:release greetings.nim By default the Nimrod compiler generates a large amount of runtime checks aiming for your debugging pleasure. With ``-d:release`` these 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 Nimrod'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: .. code-block:: Nimrod var name = readline(stdin) Note that this is basically the only form of type inference that exists in Nimrod: 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-in items are declared in the system_ module which is implicitly imported by any other module. Lexical elements ================ Let us look at Nimrod's lexical elements in more detail: like other programming languages Nimrod consists of (string) literals, identifiers, keywords, comments, operators, and other punctuation marks. Case is *insignificant* in Nimrod and even underscores are ignored: ``This_is_an_identifier`` and ``ThisIsAnIdentifier`` are the same identifier. This feature enables you to use other people's code without bothering about a naming convention that conflicts with yours. It also frees you from remembering the exact spelling of an identifier (was it ``parseURL`` or ``parseUrl`` or ``parse_URL``?). String and character literals ----------------------------- 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: .. code-block:: Nimrod r"C:\program files\nim" In raw literals the backslash is not an escape character. The third and last way to write string literals are *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 -------- `Comments`:idx: start anywhere outside a string or character literal with the hash character ``#``. Documentation comments start with ``##``. Comments consist of a concatenation of `comment pieces`:idx:. A comment piece starts with ``#`` and runs until the end of the line. The end of line characters belong to the piece. If the next line only consists of a comment piece which is aligned to the preceding one, it does not start a new comment: .. code-block:: nimrod i = 0 # This is a single comment over multiple lines belonging to the # assignment statement. The scanner merges these two pieces. # This is a new comment belonging to the current block, but to no particular # statement. i = i + 1 # This a new comment that is NOT echo(i) # continued here, because this comment refers to the echo statement Comments are tokens; they are only allowed at certain places in the input file as they belong to the syntax tree! This feature enables perfect source-to-source transformations (such as pretty-printing) and simpler documentation generators. A nice side-effect is that the human reader of the code always knows exactly which code snippet the comment refers to. Since comments are a proper part of the syntax, watch their indentation: .. code-block:: Echo("Hello!") # comment has the same indentation as above statement -> fine Echo("Hi!") # comment has not the right indentation -> syntax error! **Note**: To comment out a large piece of code, it is often better to use a ``when false:`` statement. Numbers ------- 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 million). 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 ================= The var statement declares a new local or global variable: .. code-block:: var x, y: int # declares x and y to have the type ``int`` Indentation can be used after the ``var`` keyword to list a whole section of variables: .. code-block:: var x, y: int # a comment can occur here too a, b, c: string The assignment statement ======================== The assignment statement assigns a new value to a variable or more generally to a storage location: .. code-block:: var x = "abc" # introduces a new variable `x` and assigns a value to it x = "xyz" # assigns a new value to `x` ``=`` is the *assignment operator*. The assignment operator cannot be overloaded, overwritten or forbidden, but this might change in a future version of Nimrod. Constants ========= `Constants`:idx: 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: .. code-block:: nimrod const x = "abc" # the constant x contains the string "abc" Indentation can be used after the ``const`` keyword to list a whole section of constants: .. code-block:: const x = 1 # a comment can occur here too y = 2 z = y + 5 # computations are possible Control flow statements ======================= 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. If statement ------------ The if statement is one way to branch the control flow: .. code-block:: nimrod var name = readLine(stdin) if name == "": echo("Poor soul, you lost your name?") elif name == "name": echo("Very funny, your name is name.") else: Echo("Hi, ", name, "!") 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.) Case statement -------------- Another way to branch is provided by the case statement. A case statement is a multi-branch: .. code-block:: nimrod var name = readLine(stdin) case name of "": echo("Poor soul, you lost your name?") of "name": echo("Very funny, your name is name.") of "Dave", "Frank": echo("Cool name!") else: Echo("Hi, ", name, "!") 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: .. code-block:: nimrod # this statement will be explained later: from strutils import parseInt Echo("A number please: ") var n = parseInt(readLine(stdin)) case n of 0..2, 4..7: Echo("The number is in the set: {0, 1, 2, 4, 5, 6, 7}") of 3, 8: Echo("The number is 3 or 8") 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: .. code-block:: nimrod ... case n of 0..2, 4..7: Echo("The number is in the set: {0, 1, 2, 4, 5, 6, 7}") of 3, 8: Echo("The number is 3 or 8") else: nil The ``nil`` 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 there is no such check for string cases. 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. While statement --------------- The while statement is a simple looping construct: .. code-block:: nimrod Echo("What's your name? ") var name = readLine(stdin) while name == "": Echo("Please tell me your name: ") name = readLine(stdin) # no ``var``, because we do not declare a new variable here The example uses a while loop to keep asking the user for his name, as long as he types in nothing (only presses RETURN). For statement ------------- The `for`:idx: statement is a construct to loop over any element an *iterator* provides. The example uses the built-in ``countup`` iterator: .. code-block:: nimrod Echo("Counting to ten: ") for i in countup(1, 10): Echo($i) The built-in ``$`` operator turns an integer (``int``) and many other types into a string. 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: .. code-block:: nimrod Echo("Counting to 10: ") var i = 1 while i <= 10: Echo($i) inc(i) # increment i by 1 Counting down can be achieved as easily (but is less often needed): .. code-block:: nimrod Echo("Counting down from 10 to 1: ") for i in countdown(10, 1): Echo($i) Since counting up occurs so often in programs, Nimrod has a special syntax that calls the ``countup`` iterator implicitly: .. code-block:: nimrod for i in 1..10: ... The syntax ``for i in 1..10`` is sugar for ``for i in countup(1, 10)``. ``countdown`` does not have any such sugar. Scopes and the block statement ------------------------------ 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: .. code-block:: nimrod while false: var x = "hi" echo(x) # does not work 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: .. code-block:: nimrod block myblock: var x = "hi" echo(x) # does not work either The block's `label` (``myblock`` in the example) is optional. Break statement --------------- 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: .. code-block:: nimrod block myblock: Echo("entering block") while true: Echo("looping") break # leaves the loop, but not the block Echo("still in block") block myblock2: Echo("entering block") while true: Echo("looping") break myblock2 # leaves the block (and the loop) Echo("still in block") Continue statement ------------------ Like in many other programming languages, a ``continue`` statement starts the next iteration immediately: .. code-block:: nimrod while true: var x = readLine(stdin) if x == "": continue Echo(x) When statement -------------- Example: .. code-block:: nimrod when system.hostOS == "windows": echo("running on Windows!") elif system.hostOS == "linux": echo("running on Linux!") elif system.hostOS == "macosx": echo("running on Mac OS X!") else: echo("unknown operating system") The `when`:idx: statement is almost identical to the ``if`` statement with some differences: * Each condition has to 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. **Note**: The documentation generator currently always follows the first branch of when statements. **Note**: To comment out a large piece of code, it is often better to use a ``when false:`` statement than to use real comments. This way nesting is possible. Statements and indentation ========================== Now that we covered the basic control flow statements, let's return to Nimrod indentation rules. In Nimrod there is a distinction between *simple statements* and *complex statements*. *Simple statements* cannot contain other statements: Assignment, procedure calls or the ``return`` statement belong to the simple statements. *Complex statements* like ``if``, ``when``, ``for``, ``while`` can contain other statements. To avoid ambiguities, complex statements always have to be indented, but single simple statements do not: .. code-block:: nimrod # no indentation needed for single assignment statement: if x: x = false # indentation needed for nested if statement: if x: if y: y = false else: y = true # indentation needed, because two statements follow the condition: if x: x = false y = false *Expressions* are parts of a statement which usually result in a value. The condition in an if statement is an example for an expression. Expressions can contain indentation at certain places for better readability: .. code-block:: nimrod if thisIsaLongCondition() and thisIsAnotherLongCondition(1, 2, 3, 4): x = true As a rule of thumb, indentation within expressions is allowed after operators, an open parenthesis and after commas. Procedures ========== To define new commands like ``echo``, ``readline`` in the examples, the concept of a `procedure` is needed. (Some languages call them *methods* or *functions*.) In Nimrod new procedures are defined with the ``proc`` keyword: .. code-block:: nimrod proc yes(question: string): bool = Echo(question, " (y/n)") while true: case readLine(stdin) of "y", "Y", "yes", "Yes": return true of "n", "N", "no", "No": return false else: Echo("Please be clear: yes or no") if yes("Should I delete all your important files?"): Echo("I'm sorry Dave, I'm afraid I can't do that.") else: Echo("I think you know what the problem is just as well as I do.") This example shows a procedure named ``yes`` that asks the user a ``question`` and returns true if he answered "yes" (or something similar) and returns false if he 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``. ``Bool`` is a built-in type: the only valid values for ``bool`` are ``true`` and ``false``. The conditions in if or while statements should be of the 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. Result variable --------------- A procedure that returns a value has an implicit ``result`` variable that represents the return value. A ``return`` statement with no expression is a shorthand for ``return result``. So all three code snippets are equivalent: .. code-block:: nimrod return 42 .. code-block:: nimrod result = 42 return .. code-block:: nimrod result = 42 return result Parameters ---------- Parameters are constant in the procedure body. Their value cannot be changed because this allows the compiler to implement parameter passing in the most efficient way. If the procedure needs to modify the argument for the caller, a ``var`` parameter can be used: .. code-block:: nimrod proc divmod(a, b: int, res, remainder: var int) = res = a div b remainder = a mod b var x, y: int divmod(8, 5, x, y) # modifies x and y echo(x) echo(y) 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. Discard statement ----------------- To call a procedure that returns a value just for its side effects and ignoring its return value, a discard statement **has** to be used. Nimrod does not allow to silently throw away a return value: .. code-block:: nimrod discard yes("May I ask a pointless question?") Named arguments --------------- Often a procedure has many parameters and it is not clear in which order the parameters appeared. 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: .. code-block:: nimrod proc createWindow(x, y, width, height: int, title: string, show: bool): Window = ... var w = createWindow(show = true, title = "My Application", x = 0, y = 0, height = 600, width = 800) 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: .. code-block:: nimrod var w = createWindow(0, 0, title = "My Application", height = 600, width = 800, true) The compiler checks that each parameter receives exactly one argument. Default values -------------- 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: .. code-block:: nimrod proc createWindow(x = 0, y = 0, width = 500, height = 700, title = "unknown", show = true): Window = ... var w = createWindow(title = "My Application", height = 600, width = 800) 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. Overloaded procedures --------------------- Nimrod provides the ability to overload procedures similar to C++: .. code-block:: nimrod proc toString(x: int): string = ... proc toString(x: bool): string = if x: return "true" else: return "false" Echo(toString(13)) # calls the toString(x: int) proc Echo(toString(true)) # calls the toString(x: bool) proc (Note that ``toString`` is usually the ``$`` operator in Nimrod.) 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. Operators --------- The Nimrod library makes heavy use of overloading - one reason for this is that each operator like ``+`` is a 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 Nimrod. 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 readability can suffer. 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 "``": .. code-block:: nimrod proc `$` (x: myDataType): string = ... # now the $ operator also works with myDataType, overloading resolution # ensures that $ works for built-in types just like before The "``" notation can also be used to call an operator just like a procedure with a real name: .. code-block:: nimrod if `==`( `+`(3, 4), 7): Echo("True") Forward declarations -------------------- Every variable, procedure, etc. needs to be declared before it can be used. (The reason for this is compilation efficiency.) However, this cannot be done for mutually recursive procedures: .. code-block:: nimrod # forward declaration: proc even(n: int): bool proc odd(n: int): bool = if n == 1: return true else: return even(n-1) proc even(n: int): bool = if n == 0: return true else: return odd(n-1) 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. Iterators ========= Let's return to the boring counting example: .. code-block:: nimrod Echo("Counting to ten: ") for i in countup(1, 10): Echo($i) Can a ``countup`` proc be written that supports this loop? Lets try: .. code-block:: nimrod proc countup(a, b: int): int = var res = a while res <= b: return res inc(res) 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 there it is - our first iterator: .. code-block:: nimrod iterator countup(a, b: int): int = var res = a while res <= b: yield res inc(res) Iterators look very similar to procedures, but there are several important differences: * Iterators can only be called from for loops. * Iterators cannot contain a ``return`` statement and procs cannot contain a ``yield`` statement. * Iterators have no implicit ``result`` variable. * Iterators do not support recursion. (This restriction will be gone in a future version of the compiler.) * 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.) Basic types =========== This section deals with the basic built-in types and the operations that are available for them in detail. Booleans -------- The `boolean`:idx: type is named ``bool`` in Nimrod and consists of the two pre-defined values ``true`` and ``false``. Conditions in while, if, elif, when statements need to be of type bool. The operators ``not, and, or, xor, <, <=, >, >=, !=, ==`` are defined for the bool type. The ``and`` and ``or`` operators perform short-cut evaluation. Example: .. code-block:: nimrod while p != nil and p.name != "xyz": # p.name is not evaluated if p == nil p = p.next Characters ---------- The `character type` is named ``char`` in Nimrod. Its size is one byte. Thus it cannot represent an UTF-8 character, but a part of it. 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 specially 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. Strings ------- String variables in Nimrod are **mutable**, so appending to a string is quite efficient. Strings in Nimrod are both zero-terminated and have a length field. One can retrieve a string's length with the builtin ``len`` procedure; the length never counts the terminating zero. Accessing the terminating zero is no error and often leads to simpler code: .. code-block:: nimrod if s[i] == 'a' and s[i+1] == 'b' and s[i+2] == '\0': # no need to check whether ``i < len(s)``! ... The assignment operator for strings copies the string. You can use the ``&`` operator to concatenate strings. Strings are compared by their lexicographical order. All comparison operators are available. Per convention, all strings are UTF-8 strings, 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*. String variables are initialized with a special value, called ``nil``. However, most string operations cannot deal with ``nil`` (leading to an exception being raised) for performance reasons. One should use empty strings ``""`` rather than ``nil`` as the *empty* value. But ``""`` often creates a string object on the heap, so there is a trade-off to be made here. Integers -------- Nimrod has these integer types built-in: ``int int8 int16 int32 int64``. These are all signed integer types, there are no `unsigned integer`:idx: types, only `unsigned operations`:idx: that treat their arguments as unsigned. The default integer type is ``int``. Integer literals can have a *type suffix* to mark them to be of another integer type: .. code-block:: nimrod var x = 0 # x is of type ``int`` y = 0'i8 # y is of type ``int8`` z = 0'i64 # z is of type ``int64`` 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 defined for integers too 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 underflow errors. Unsigned operations use the ``%`` suffix as convention: ====================== ====================================================== operation meaning ====================== ====================================================== ``a +% b`` unsigned integer addition ``a -% b`` unsigned integer subtraction ``a *% b`` unsigned integer multiplication ``a /% b`` unsigned integer division ``a %% b`` unsigned integer modulo operation ``a <% b`` treat ``a`` and ``b`` as unsigned and compare ``a <=% b`` treat ``a`` and ``b`` as unsigned and compare ====================== ====================================================== `Automatic type conversion`:idx: is performed in expressions where different kinds of integer types are used. However, if the type conversion loses information, the `EOutOfRange`:idx: exception is raised (if the error cannot be detected at compile time). Floats ------ Nimrod has these floating point types built-in: ``float float32 float64``. The default float type is ``float``. In the current implementation, ``float`` is always 64 bit wide. Float literals can have a *type suffix* to mark them to be of another float type: .. code-block:: nimrod var x = 0.0 # x is of type ``float`` y = 0.0'f32 # y is of type ``float32`` z = 0.0'f64 # z is of type ``float64`` The common operators ``+ - * / < <= == != > >=`` are defined for floats and follow the IEEE 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 and vice versa. The ``toInt`` and ``toFloat`` procs can be used for these conversions. Advanced types ============== In Nimrod new types can be defined within a ``type`` statement: .. code-block:: nimrod type biggestInt = int64 # biggest integer type that is available biggestFloat = float64 # biggest float type that is available Enumeration and object types cannot be defined on the fly, but only within a ``type`` statement. Enumerations ------------ A variable of an `enumeration`:idx: type can only be assigned a value of a limited set. This set consists 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. Example: .. code-block:: nimrod type TDirection = enum north, east, south, west var x = south # `x` is of type `TDirection`; its value is `south` echo($x) # writes "south" to `stdout` (To prefix a new type with the letter ``T`` is a convention in Nimrod.) All comparison operators can be used with enumeration types. An enumeration's symbol can be qualified to avoid ambiguities: ``TDirection.south``. The ``$`` operator can convert any enumeration value to its name, the ``ord`` proc 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 have to be in ascending order. A symbol whose ordinal value is not explicitly given is assigned the value of the previous symbol + 1. An explicit ordered enum can have *holes*: .. code-block:: nimrod type TMyEnum = enum a = 2, b = 4, c = 89 Ordinal types ------------- Enumerations without holes, integer types, ``char`` and ``bool`` (and subranges) are called `ordinal`:idx: 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` ``prec(x)`` returns the predecessor of `x` ``pred(x, n)`` returns the `n`'th predecessor of `x` ----------------- -------------------------------------------------------- The ``inc dec succ pred`` operations can fail by raising an `EOutOfRange` or `EOverflow` exception. (If the code has been compiled with the proper runtime checks turned on.) Subranges --------- A `subrange`:idx: type is a range of values from an integer or enumeration type (the base type). Example: .. code-block:: nimrod type TSubrange = range[0..5] ``TSubrange`` is a subrange of ``int`` which can only hold the values 0 to 5. Assigning any other value to a variable of type ``TSubrange`` 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 mandate the usage of unsigned integers for natural numbers. This is often **wrong**: you don't want unsigned arithmetic (which wraps around) just because the numbers cannot be negative. Nimrod's ``natural`` type helps to avoid this common programming error. Sets ---- The `set type`:idx: models the mathematical notion of a set. The set's basetype can only be an ordinal type. The reason is that sets are implemented as high performance bit vectors. Sets can be constructed via the set constructor: ``{}`` is the empty set. The empty set is type compatible with any concrete set type. The constructor can also be used to include elements (and ranges of elements): .. code-block:: nimrod type TCharSet = set[char] var x: TCharSet x = {'a'..'z', '0'..'9'} # This constructs a set that conains the # letters from 'a' to 'z' and the digits # from '0' to '9' These operations are supported by sets: ================== ======================================================== operation meaning ================== ======================================================== ``A + B`` union of two sets ``A * B`` intersection of two sets ``A - B`` difference of two sets (A without B's elements) ``A == B`` set equality ``A <= B`` subset relation (A is subset of B or equal to B) ``A < B`` strong subset relation (A is a real subset of B) ``e in A`` set membership (A contains element e) ``e notin A`` A does not contain element e ``contains(A, e)`` A contains element e ``A -+- B`` symmetric set difference (= (A - B) + (B - A)) ``card(A)`` the cardinality of A (number of elements in A) ``incl(A, elem)`` same as ``A = A + {elem}`` ``excl(A, elem)`` same as ``A = A - {elem}`` ================== ======================================================== Sets are often used to define a type for the *flags* of a procedure. This is a much cleaner (and type safe) solution than just defining integer constants that should be ``or``'ed together. Arrays ------ An `array`:idx: is a simple fixed length container. Each element in the array has the same type. The array's index type can be any ordinal type. Arrays can be constructed via ``[]``: .. code-block:: nimrod type TIntArray = array[0..5, int] # an array that is indexed with 0..5 var x: TIntArray x = [1, 2, 3, 4, 5, 6] for i in low(x)..high(x): echo(x[i]) 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 Nimrod 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. Sequences --------- `Sequences`:idx: are similar to arrays but of dynamic length which may change during runtime (like strings). Since sequences are resizeable 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: .. code-block:: nimrod var x: seq[int] # a sequence of integers x = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence Sequence variables are initialized with ``nil``. However, most sequence operations cannot deal with ``nil`` (leading to an exception being raised) for performance reasons. Thus one should use empty sequences ``@[]`` rather than ``nil`` as the *empty* value. But ``@[]`` creates a sequence object on the heap, so there is a trade-off to be made here. Open arrays ----------- **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. An openarray 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: .. code-block:: nimrod proc myWriteln(f: TFile, a: openarray[string]) = for s in items(a): write(f, s) write(f, "\n") myWriteln(stdout, "abc", "def", "xyz") # is transformed by the compiler to: myWriteln(stdout, ["abc", "def", "xyz"]) This transformation is only done if the openarray parameter is the last parameter in the procedure header. Tuples ------ A tuple type defines various named *fields* and an *order* of the fields. 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. Different tuple-types are *equivalent* if they specify the same fields of the same type in the same order. The assignment operator for tuples copies each component. The notation ``t.field`` is used to access a tuple's field. Another notation is ``t[i]`` to access the ``i``'th field. Here ``i`` needs to be a constant integer. .. code-block:: nimrod type TPerson = tuple[name: string, age: int] # type representing a person: # a person consists of a name # and an age var person: TPerson person = (name: "Peter", age: 30) # the same, but less readable: person = ("Peter", 30) echo(person.name) # "Peter" echo(person.age) # 30 echo(person[0]) # "Peter" echo(person[1]) # 30 Reference and pointer types --------------------------- References (similar to `pointers`:idx: 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. Nimrod distinguishes between `traced`:idx: and `untraced`:idx: references. Untraced references are also called *pointers*. Traced references point to objects of a garbage collected heap, untraced references point to manually allocated objects or to objects somewhere else in memory. Thus untraced references are *unsafe*. However for certain low-level operations (accessing the hardware) untraced references are unavoidable. Traced references are declared with the **ref** keyword, untraced references are declared with the **ptr** keyword. The empty ``[]`` subscript notation can be used to *derefer* a reference, meaning to retrieve the item the reference points to. The ``addr`` procedure returns the address of an item. An address is always an untraced reference: ``addr`` is an *unsafe* feature. The ``.`` (access a tuple/object field operator) and ``[]`` (array/string/sequence index operator) operators perform implicit dereferencing operations for reference types: .. code-block:: nimrod type PNode = ref TNode TNode = tuple[le, ri: PNode, data: int] var n: PNode new(n) n.data = 9 # no need to write n[].data; in fact n[].data is highly discouraged! (As a convention, reference types use a 'P' prefix.) To allocate a new traced object, the built-in procedure ``new`` has to be used. To deal with untraced memory, the procedures ``alloc``, ``dealloc`` and ``realloc`` can be used. The documentation of the system module contains further information. If a reference points to *nothing*, it has the value ``nil``. Procedural type --------------- A `procedural type`:idx: is a (somewhat abstract) pointer to a procedure. ``nil`` is an allowed value for a variable of a procedural type. Nimrod uses procedural types to achieve `functional`:idx: programming techniques. Example: .. code-block:: nimrod type TCallback = proc (x: int) proc echoItem(x: Int) = echo(x) proc forEach(callback: TCallback) = const data = [2, 3, 5, 7, 11] for d in items(data): callback(d) forEach(echoItem) 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 `user guide"Der Mensch ist doch ein Augentier -- schöne Dinge wünsch ich mir."