
Chapter 4 OMake concepts and syntax
Projects are specified to omake with OMakefiles. 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 functional (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
form. The included file must use
the same syntax as an OMakefile.
include files.omake
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 can be used to circumvent this restriction by
exporting variable values from an inner scope. It must be the final
expression in a 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 8.2 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 $1)
default
eprintln(Machine configuration not implemented)
exit(1)
OMake is an object-oriented language. Generally speaking, an object is a value that contains fields
and methods. An object is defined with a .
suffix for a variable. For example, the
following object might be used to specify a point (1, 5) on the two-dimensional plane.
Coord. =
x = 1
y = 5
print(message) =
println($"$(message): the point is ($(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.
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 Special objects/sections
Objects provide one way to manage the OMake namespace. There are also four special objects that are
further used to control the namespace.
4.15 private.
The private.
section is used to define variables that are private to the current file/scope.
The values are not accessible outside the scope. Variables defined in a private.
object can
be accessed only from within the section where they are defined.
Obj. =
private. =
X = 1
print() =
println(The value of X is: $(X))
# Prints:
# The private value of X is: 1
Obj.print()
# This is an error--X is private in Obj
y = $(Obj.X)
In addition, private definitions do not affect the global value of a variable.
# The public value of x is 1
x = 1
f() =
println(The public value of x is: $(x))
# This object uses a private value of x
Obj. =
private. =
x = 2
print() =
x = 3
println(The private value of x is: $(x))
f()
# Prints:
# The private value of x is: 3
# The public value of x is: 1
Obj.print()
Private variables have two additional properties.
-
Private variables are local to the file in which they are defined.
- Private variables are not exported by the
export
directive, unless they are
mentioned explicitly.
private. =
FLAG = true
section
FLAG = false
export
# FLAG is still true
section
FLAG = false
export FLAG
# FLAG is now false
4.16 protected.
The protected.
object is used to define fields that are local to an object. They can
be accessed as fields, but they are not passed dynamically to other functions. The purpose of a
protected variable is to prevent a variable definition within the object from affecting other parts
of the project.
X = 1
f() =
println(The public value of X is: $(X))
# Prints:
# The public value of X is: 2
section
X = 2
f()
# X is a protected field in the object
Obj. =
protected. =
X = 3
print() =
println(The protected value of X is: $(X))
f()
# Prints:
# The protected value of X is: 3
# The public value of X is: 1
Obj.print()
# This is legal, it defines Y as 3
Y = $(Obj.X)
In general, it is a good idea to define object variables as protected. The resulting code is more
modular because variables in your object will not produce unexpected clashes with variables defined
in other parts of the project.
The public.
object is used to specify public dynamically-scoped variables. In the following
example, the public.
object specifies that the value X = 4
is to be dynamically
scoped. Public variables are not defined as fields of an object.
X = 1
f() =
println(The public value of X is: $(X))
# Prints:
# The public value of X is: 2
section
X = 2
f()
Obj. =
protected. =
X = 3
print() =
println(The protected value of X is: $(X))
public. =
X = 4
f()
# Prints:
# The protected value of X is: 3
# The public value of X is: 4
Obj.print()
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)')
As a matter of style, a static.
section that is used for configuration should print what it
is doing, using ---
as a print prefix.
4.19 Short syntax for scoping objects
The usual dot-notation can be used for private, protected, and public variables (but not
static variables).
# Public definition of X
public.X = 1
# Private definition of X
private.X = 2
# Prints:
# The public value of X is: 1
# The private value of X is: 2
println(The public value of X is: $(public.X))
println(The private value of X is: $(private.X))
4.20 Modular programming
The scoping objects help provide a form of modularity. When you write a new file or program,
explicit scoping declarations can be used to define an explicit interface for your code, and help
avoid name clashes with other parts of the project. Variable definitions are public by default, but
you can control this with private definitions.
# These variables are private to this file
private. =
FILES = foo1 foo2 foo3
SUFFIX = .o
OFILES = $(addsuffix $(SUFFIX), $(FILES))
# These variables are public
public. =
CFLAGS += -g
# Build the files with the -g option
$(OFILES):