OMake concepts and syntax

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# Chapter 4 OMake concepts and syntax

- Variables
- Adding to a variable definition
- Arrays
- Special characters and quoting
- Function definitions
- Comments
- File inclusion
- Scoping, sections
- Conditionals
- Matching
- Objects
- Classes
- Inheritance
- static.
- Constants

Projects are specified to `omake` with `OMakefile`s. The `OMakefile` has a format similar to a `Makefile`. An `OMakefile` has three main kinds of syntactic objects: variable definitions, function definitions, and rule definitions.

## 4.1 Variables

Variables are defined with the following syntax. The name is any sequence of alphanumeric characters, underscore `_`, and hyphen `-`.

       <name> = <value>

Values are defined as a sequence of literal characters and variable expansions. A variable expansion has the form `$(<name>)`, which represents the value of the `<name>` variable in the current environment. Some examples are shown below.

       CC = gcc
       CFLAGS = -Wall -g
       COMMAND = $(CC) $(CFLAGS) -O2

In this example, the value of the `COMMAND` variable is the string `gcc -Wall -g -O2`.

Unlike `make`(1), variable expansion is eager and pure (see also the section on Scoping). That is, variable values are expanded immediately and new variable definitions do not affect old ones. For example, suppose we extend the previous example with following variable definitions.

       X = $(COMMAND)
       COMMAND = $(COMMAND) -O3
       Y = $(COMMAND)

In this example, the value of the `X` variable is the string `gcc -Wall -g -O2` as before, and the value of the `Y` variable is `gcc -Wall -g -O2 -O3`.

## 4.2 Adding to a variable definition

Variables definitions may also use the += operator, which adds the new text to an existing definition. The following two definitions are equivalent.

       # Add options to the CFLAGS variable
       CFLAGS = $(CFLAGS) -Wall -g

       # The following definition is equivalent
       CFLAGS += -Wall -g

## 4.3 Arrays

Arrays can be defined by appending the `[]` sequence to the variable name and defining initial values for the elements as separate lines. Whitespace is significant on each line. The following code sequence prints `c d e`.

        X[] =
            a b
            c d e
            f

        println($(nth 2, $(X)))

## 4.4 Special characters and quoting

The following characters are special to `omake`: `$():,=#\`. To treat any of these characters as normal text, they should be escaped with the backslash character `\`.

        DOLLAR = \$

Newlines may also be escaped with a backslash to concatenate several lines.

        FILES = a.c\
                b.c\
                c.c

Note that the backslash is not an escape for any other character, so the following works as expected (that is, it preserves the backslashes in the string).

        DOSTARGET = C:\WINDOWS\control.ini

An alternative mechanism for quoting special text is the use `$"..."` escapes. The number of double-quotations is arbitrary. The outermost quotations are not included in the text.

        A = $""String containing "quoted text" ""
        B = $"""Multi-line
            text.
            The # character is not special"""

## 4.5 Function definitions

Functions are defined using the following syntax.

       <name>(<params>) =
          <indented-body>

The parameters are a comma-separated list of identifiers, and the body must be placed on a separate set of lines that are indented from the function definition itself. For example, the following text defines a function that concatenates its arguments, separating them with a colon.

        ColonFun(a, b) =
            return($(a):$(b))

The `return` expression can be used to return a value from the function. A `return` statement is not required; if it is omitted, the returned value is the value of the last expression in the body to be evaluated. NOTE: as of version `0.9.6`, `return` is a control operation, causing the function to immediately return. In the following example, when the argument `a` is true, the function `f` immediately returns the value 1 without evaluating the print statement.

        f(a) =
           if $(a)
              return 1
           println(The argument is false)
           return 0

In many cases, you may wish to return a value from a section or code block without returning from the function. In this case, you would use the `value` operator. In fact, the `value` operator is not limited to functions, it can be used any place where a value is required. In the following definition, the variable `X` is defined as 1 or 2, depending on the value of a, then result is printed, and returned from the function.

        f_value(a) =
           X =
              if $(a)
                 value 1
              else
                 value 2
           println(The value of X is $(X))
           value $(X)

Functions are called using the GNU-make syntax, `$(<name> <args))`, where `<args>` is a comma-separated list of values. For example, in the following program, the variable `X` contains the value `foo:bar`.

       X = $(ColonFun foo, bar)

If the value of a function is not needed, the function may also be called using standard function call notation. For example, the following program prints the string “She says: Hello world”.

        Printer(name) =
            println($(name) says: Hello world)

        Printer(She)

## 4.6 Comments

Comments begin with the `#` character and continue to the end of the line.

## 4.7 File inclusion

Files may be included with the `include` or `open` form. The included file must use the same syntax as an `OMakefile`.

        include $(Config_file)

The `open` operation is similar to an `include`, but the file is included at most once.

        open Config

        # Repeated opens are ignored, so this
        # line has no effect.
        open Config

If the file specified is not an absolute filenmame, both `include` and `open` operations search for the file based on the `OMAKEPATH` variable. In case of the `open` directive, the search is performed at parse time, and the argument to `open` may not contain any expressions.

## 4.8 Scoping, sections

Scopes in `omake` are defined by indentation level. When indentation is increased, such as in the body of a function, a new scope is introduced.

The `section` form can also be used to define a new scope. For example, the following code prints the line `X = 2`, followed by the line `X = 1`.

        X = 1
        section
            X = 2
            println(X = $(X))

        println(X = $(X))

This result may seem surprising–the variable definition within the `section` is not visible outside the scope of the `section`.

The `export` form, which will be described in detail in Section 6.3, can be used to circumvent this restriction by exporting variable values from an inner scope. For example, if we modify the previous example by adding an `export` expression, the new value for the `X` variable is retained, and the code prints the line `X = 2` twice.

        X = 1
        section
            X = 2
            println(X = $(X))
            export

        println(X = $(X))

There are also cases where separate scoping is quite important. For example, each `OMakefile` is evaluated in its own scope. Since each part of a project may have its own configuration, it is important that variable definitions in one `OMakefile` do not affect the definitions in another.

To give another example, in some cases it is convenient to specify a separate set of variables for different build targets. A frequent idiom in this case is to use the `section` command to define a separate scope.

       section
          CFLAGS += -g
          %.c: %.y
              $(YACC) $<
          .SUBDIRS: foo

       .SUBDIRS: bar baz

In this example, the `-g` option is added to the `CFLAGS` variable by the `foo` subdirectory, but not by the `bar` and `baz` directories. The implicit rules are scoped as well and in this example, the newly added yacc rule will be inherited by the `foo` subdirectory, but not by the `bar` and `baz` ones; furthermore this implicit rule will not be in scope in the current directory.

## 4.9 Conditionals

Top level conditionals have the following form.

        if <test>
           <true-clause>
        elseif <text>
           <elseif-clause>
        else
           <else-clause>

The `<test>` expression is evaluated, and if it evaluates to a true value (see Section 9.2 for more information on logical values, and Boolean functions), the code for the `<true-clause>` is evaluated; otherwise the remaining clauses are evaluated. There may be multiple `elseif` clauses; both the `elseif` and `else` clauses are optional. Note that the clauses are indented, so they introduce new scopes.

When viewed as a predicate, a value corresponds to the Boolean false, if its string representation is the empty string, or one of the strings `false`, `no`, `nil`, `undefined`, or `0`. All other values are true.

The following example illustrates a typical use of a conditional. The `OSTYPE` variable is the current machine architecture.

        # Common suffixes for files
        if $(equal $(OSTYPE), Win32)
           EXT_LIB = .lib
           EXT_OBJ = .obj
           EXT_ASM = .asm
           EXE = .exe
           export
        elseif $(mem $(OSTYPE), Unix Cygwin)
           EXT_LIB = .a
           EXT_OBJ = .o
           EXT_ASM = .s
           EXE =
           export
        else
           # Abort on other architectures
           eprintln(OS type $(OSTYPE) is not recognized)
           exit(1)

## 4.10 Matching

Pattern matching is performed with the `switch` and `match` forms.

        switch <string>
        case <pattern1>
            <clause1>
        case <pattern2>
            <clause2>
        ...
        default
           <default-clause>

The number of cases is arbitrary. The `default` clause is optional; however, if it is used it should be the last clause in the pattern match.

For `switch`, the string is compared with the patterns literally.

        switch $(HOST)
        case mymachine
            println(Building on mymachine)
        default
            println(Building on some other machine)

Patterns need not be constant strings. The following function tests for a literal match against `pattern1`, and a match against `pattern2` with `##` delimiters.

       Switch2(s, pattern1, pattern2) =
          switch $(s)
          case $(pattern1)
              println(Pattern1)
          case $"##$(pattern2)##"
              println(Pattern2)
          default
              println(Neither pattern matched)

For `match` the patterns are `egrep`(1)-style regular expressions. The numeric variables `$1, $2, ...` can be used to retrieve values that are matched by `\(...\)` expressions.

        match $(NODENAME)@$(SYSNAME)@$(RELEASE)
        case $"mymachine.*@\(.*\)@\(.*\)"
            println(Compiling on mymachine; sysname $1 and release $2 are ignored)

        case $".*@Linux@.*2\.4\.\(.*\)"
            println(Compiling on a Linux 2.4 system; subrelease is $1)

        default
            eprintln(Machine configuration not implemented)
            exit(1)

## 4.11 Objects

OMake is an object-oriented language. Generally speaking, an object is a value that contains fields and methods. An object is defined with a `.` suffix for a variable. For example, the following object might be used to specify a point (1, 5) on the two-dimensional plane.

        Coord. =
            x = 1
            y = 5
            print(message) =
               println($"$(message): the point is ($(x), $(y)")

        # Define X to be 5
        X = $(Coord.x)

        # This prints the string, "Hi: the point is (1, 5)"
        Coord.print(Hi)

The fields `x` and `y` represent the coordinates of the point. The method `print` prints out the position of the point.

## 4.12 Classes

We can also define classes. For example, suppose we wish to define a generic `Point` class with some methods to create, move, and print a point. A class is really just an object with a name, defined with the `class` directive.

        Point. =
            class Point

            # Default values for the fields
            x = 0
            y = 0

            # Create a new point from the coordinates
            new(x, y) =
               this.x = $(x)
               this.y = $(y)
               return $(this)

            # Move the point to the right
            move-right() =
               x = $(add $(x), 1)
               return $(this)

            # Print the point
            print() =
               println($"The point is ($(x), $(y)")

        p1 = $(Point.new 1, 5)
        p2 = $(p1.move-right)

        # Prints "The point is (1, 5)"
        p1.print()

        # Prints "The point is (2, 5)"
        p2.print()

Note that the variable `$(this)` is used to refer to the current object. Also, classes and objects are functional—the `new` and `move-right` methods return new objects. In this example, the object `p2` is a different object from `p1`, which retains the original (1, 5) coordinates.

## 4.13 Inheritance

Classes and objects support inheritance (including multiple inheritance) with the `extends` directive. The following definition of `Point3D` defines a point with `x`, `y`, and `z` fields. The new object inherits all of the methods and fields of the parent classes/objects.

        Z. =
           z = 0

        Point3D. =
           extends $(Point)
           extends $(Z)
           class Point3D

           print() =
              println($"The 3D point is ($(x), $(y), $(z))")

        # The "new" method was not redefined, so this
        # defines a new point (1, 5, 0).
        p = $(Point3D.new 1, 5)

## 4.14 static.

The `static.` object is used to specify values that are persistent across runs of OMake. They are frequently used for configuring a project. Configuring a project can be expensive, so the `static.` object ensure that the configuration is performed just once. In the following (somewhat trivial) example, a `static` section is used to determine if the LATEX command is available. The `$(where latex)` function returns the full pathname for `latex`, or `false` if the command is not found.

       static. =
          LATEX_ENABLED = false
          print(--- Determining if LaTeX is installed )
          if $(where latex)
              LATEX_ENABLED = true
              export

          if $(LATEX_ENABLED)
             println($'(enabled)')
          else
             println($'(disabled)')

The OMake standard library provides a number of useful functions for programming the `static.` tests, as described in Chapter 14. Using the standard library, the above can be rewritten as

       open configure/Configure
       static. =
          LATEX_ENABLED = $(CheckProg latex)

As a matter of style, a `static.` section that is used for configuration should print what it is doing using the `ConfMsgChecking` and `ConfMsgResult` functions (of course, most of helper functions in the standard library would do that automatically).

### 4.14.1 .STATIC

This feature was introduced in version 0.9.8.5.

There is also a rule form of static section. The syntax can be any of the following three forms.

        # Export all variables defined by the body
        .STATIC:
            <body>

        # Specify file-dependencies
        .STATIC: <dependencies>
            <body>

        # Specify which variables to export, as well as file dependencies
        .STATIC: <vars>: <dependencies>
            <body>

The `<vars>` are the variable names to be defined, the `<dependencies>` are file dependencies—the rule is re-evaluated if one of the dependencies is changed. The `<vars>` and `<dependencies>` can be omitted; if so, all variables defined in the `<body>` are exported.

For example, the final example of the previous section can also be implemented as follows.

        open configure/Configure
        .STATIC:
            LATEX_ENABLED = $(CheckProg latex)

The effect is much the same as using `static.` (instead of `.STATIC`). However, in most cases `.STATIC` is preferred, for two reasons.

First, a `.STATIC` section is lazy, meaning that it is not evaluated until one of its variables is resolved. In this example, if `$(LATEX_ENABLED)` is never evaluated, the section need never be evaluated either. This is in contrast to the `static.` section, which always evaluates its body at least once.

A second reason is that a `.STATIC` section allows for file dependencies, which are useful when the `.STATIC` section is used for memoization. For example, suppose we wish to create a dictionary from a table that has key-value pairs. By using a `.STATIC` section, we can perform this computation only when the input file changes (not on every fun of `omake`). In the following example the `awk` function is used to parse the file `table-file`. When a line is encountered with the form key`=` value, the key/value pair is added the the `TABLE`.

        .STATIC: table-file
            TABLE = $(Map)
            awk(table-file)
            case $'^\([[:alnum:]]+\) *= *\(.*\)'
                TABLE = $(TABLE.add $1, $2)
                export

It is appropriate to think of a `.STATIC` section as a rule that must be recomputed whenever the dependencies of the rule change. The targets of the rule are the variables it exports (in this case, the `TABLE` variable).

#### 4.14.1.1 .MEMO

A `.MEMO` rule is just like a `.STATIC` rule, except that the results are not saved between independent runs of `omake`.

#### 4.14.1.2 :key:

The `.STATIC` and `.MEMO` rules also accept a `:key:` value, which specifies a “key” associated with the values being computed. It is useful to think of a `.STATIC` rule as a dictionary that associates keys with their values. When a `.STATIC` rule is evaluated, the result is saved in the table with the `:key:` defined by the rule (if a `:key:` is not specified, a default key is used instead). In other words, a rule is like a function. The `:key:` specifies the function “argument”, and the rule body computes the result.

To illustrate, let's use a `.MEMO` rule to implement a Fibonacci function.

        fib(i) =
            i = $(int $i)
            .MEMO: :key: $i
                println($"Computing fib($i)...")
                result =
                    if $(or $(eq $i, 0), $(eq $i, 1))
                        value $i
                    else
                        add($(fib $(sub $i, 1)), $(fib $(sub $i, 2)))
            value $(result)

        println($"fib(10) = $(fib 10)")
        println($"fib(12) = $(fib 12)")

When this script is run, it produces the following output.

        Computing fib(10)...
        Computing fib(9)...
        Computing fib(8)...
        Computing fib(7)...
        Computing fib(6)...
        Computing fib(5)...
        Computing fib(4)...
        Computing fib(3)...
        Computing fib(2)...
        Computing fib(1)...
        Computing fib(0)...
        fib(10) = 55
        Computing fib(12)...
        Computing fib(11)...
        fib(12) = 144

Note that the Fibonacci computation is performed just once for each value of the argument, rather than an exponential number of times. In other words, the `.MEMO` rule has performed a memoization, hence the name. Note that if `.STATIC` were used instead, the values would be saved across runs of `omake`.

As a general guideline, whenever you use a `.STATIC` or `.MEMO` rule within a function body, you will usually want to use a `:key:` value to index the rule by the function argument. However, this is not required. In the following, the `.STATIC` rule is used to perform some expensive computation once.

        f(x) =
            .STATIC:
                y = $(expensive-computation)
            add($x, $y)

Additonal care should be taken for recursive functions, like the Fibonacci function. If the `:key:` is omitted, then the rule would be defined in terms of itself, resulting in a cyclic dependency. Here is the output of the Fibonacci program with an omitted `:key:`.

        Computing fib(10)...
        Computing fib(8)...
        Computing fib(6)...
        Computing fib(4)...
        Computing fib(2)...
        Computing fib(0)...
        fib(10) = 0
        fib(12) = 0

The reason for this behavior is that the `result` value is not saved until the base case `i = 0 || i = 1` is reached, so `fib` calls itself recursively until reaching `fib(0)`, whereupon the `result` value is fixed at 0.

In any case, recursive definitions are perfectly acceptable, but you will usually want a `:key:` argument so that each recursive call has a different `:key:`. In most cases, this means that the `:key:` should include all arguments to the function.

## 4.15 Constants

Internally, OMake represents values in several forms, which we list here.

- int
  - Constructor: `$(int <i>)` 9.4.1.
  - Object: `Int` 12.1.4.
  - An integer is a represented with finite precision using the OCaml representation (31 bits on a 32 platform, and 63 bits on a 64 bit platform).
  - See also: arithmetic 9.4.3.
- float
  - Constructor: `$(float <x>)` 9.4.2.
  - Object: `Float` 12.1.5.
  - A float is a floating-point value, represented in IEEE 64-bit format.
  - See also: arithmetic 9.4.3.
- array
  - Constructor: `$(array <v1>, ..., <vn>)` 9.3.1.
  - Object: `Array` 12.1.7.
  - An array is a finite list of values. Arrays are also defined with an array definition

            X[] =
                <v1>
                ...
                <vn>

  - See also: `nth` 9.3.5, `nth-tl` 9.3.8, `length` 9.3.4, …
- string
  - Object: `String` 12.1.8.
  - By default, all constant character sequences represent strings, so the simple way to construct a string is to write it down. Internally, the string may be parsed as several pieces. A string often represents an array of values separated by whitespace.

            osh>S = This is a string
            - : <sequence
               "This" : Sequence
               ' ' : White
               "is" : Sequence
               ' ' : White
               "a" : Sequence
               ' ' : White
               "string" : Sequence>
               : Sequence
            osh>length($S)
            - : 4 : Int

  - A data string is a string where whitespace is significant. It represents a single value, not an array. The constructors are the quotations `$"..."` and `$'...'`.

            osh>S = $'''This is a string'''
            - : <data "This is a string"> : String

  - See also: Quoted strings 7.2.
- file
  - Constructor: `$(file <names>)` 10.1.1.
  - Object: `File` 12.1.13.
  - A file object represents the abstract name for a file. The file object can be viewed as an absolute name; the string representation depends on the current directory.

            osh>name = $(file foo)
            - : /Users/jyh/projects/omake/0.9.8.x/foo : File
            osh>echo $(name)
            foo
            osh>cd ..
            - : /Users/jyh/projects/omake : Dir
            osh>echo $(name)
            0.9.8.x/foo

  - See also: `vmount` 10.6.1.
- directory
  - Constructor: `$(dir <names>)` 10.1.1.
  - Object: `Dir` 12.1.14.
  - A directory object is like a file object, but it represents a directory.
- map (dictionary)
  - Object: `Map` 12.1.2.
  - A map/dictionary is a table that maps values to values. The `Map` object is the empty map. The data structure is persistent, and all operations are pure and functional. The special syntax `$|key|` can be used for keys that are strings.

            osh>table = $(Map)
            osh>table = $(table.add x, int)
            osh>table. +=
                    $|y| = int
            osh>table.find(y)
            - : "int" : Sequence

- channel
  - Constructor: `$(fopen <filename>, <mode>)` 10.8.4.
  - Objects: `InChannel` 12.1.16, `OutChannel` 12.1.17.
  - Channels are used for buffered input/output.
- function
  - Constructor: `$(fun <params>, <body>)` 9.5.1.
  - Object: `Fun` 12.1.9.
  - Functions can be defined in several ways.
    - As an anonymous function,

              $(fun i, j, $(add $i, $j))

    - As a named function,

              f(i, j) =
                  add($i, $j)

    - This feature will be introduced in version 0.9.9.0.As an anonymous function argument.

              osh>foreach(i => $(add $i, 1), 1 2 3)
              - : <array 2 3 4> : Array

- lexer
  - Object: `Lexer` 10.11.9.
  - This object represents a lexer.
- parser
  - Object: `Parser` 10.11.13.
  - This object represents a parser.

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