Changes between Initial Version and Version 1 of Variables

Timestamp:

04/11/12 08:51:41 (12 years ago)

Author:

hopscc

Comment:

--

Variables

@@ +1 @@
+= Variables = 
+
+Cobra supports the normal range of variables
+    * Local Variables (lexically scoped)
+    * Instance Variables
+    * Method parameter Variables
+    * Properties
+    * Class or shared/static Variables
+ 
+Variables are statically typed but typing can be done explicitly (and optionally) with a declarative
+"as clause" otherwise the variables type is inferred from initialization or first assignment or use.
+Static typing allows cobra to be fast in execution, Type inference makes static typing bearable.
+
+
+Variables are normally 'early bound' - typed at (first) use or declaration.[[BR]]
+Variables can be dynamically typed (untyped) or 'late bound' by explicitly giving them 
+(or for some variables allowing them to default to) the '''dynamic''' type.
+
+
+== Local Variables ==
+
+Local variables are created and used within a method. [[BR]
+They are used for temporary storage or calculation and are named starting with a leading 
+lowercase letter (no leading underscores or punctuation).
+
+They come into existance on first assignment unless explicitly declared
+but are created local to the '''method''' they appear in (not just the block they
+might have been first used in). [[BR]]
+A compiler error will be generated if a local variable is used (read) before it
+is created (initialized).
+
+Local variables can be explicitly typed if desired otherwise they infer their type from the
+type of what they are assigned from.
+
+{{{
+s = "name:" # s is a String since thats what it is assigned from
+greet = 'hello" + 'Mike.' # also a String since the assignment is a (String) expression
+
+i = 99 # i is an int (int32)  since thats what 99 is
+iu = 99_u8    # iu is an unsigned 8 bit int from the literal suffix on 99_u8
+
+s1 as String = 'xxx' # redundant 'as TYPE' clause since assigned from a String anyway
+
+i1 as int            # declaration without initialisation - defaults to the default value for the type (0 in this case)  
+
+
+}}}
+
+
+Local variables of Reference types should generally be assigned to, to create and type them.
+{{{
+     s as String  # generates a compiler error   
+     # needs to be
+     # s as String?
+ 
+     s = 'string value'
+     print s
+}}}
+
+Non-primitive (Reference) typed variables, if declared and not initialised [[BR]]
+need to be declared as nilable since they will default to  the default-value for [[BR]]
+a reference type which is nil (null) and if they're not nilable they can't have a nil value.
+
+Idiomatically in cobra it's best to create local variables by first use (an assignment) [[BR]] 
+rather than an explicit type declaration.
+
+
+Sometimes though you need a variable to be typed differently (usually wider) than what it is [[BR]] 
+assigned from. To do this you can explicitly type the variable with an 'as clause' on first [[BR]]
+use or cast (upcast) the initial value to the desired type.
+
+{{{
+space = 32_u8
+ispace as int32 = space
+# or
+ispace = space to int32
+
+enclosure as Shape = Circle()
+# or
+enclosure = Circle() to Shape
+}}}
+
+
+== Instance Variables ==
+
+Instance variables are variables associated with a class instance. [[BR]]
+They encapsulate the state of the instance object [[BR]] 
+They must be declared (using the '''var''' keyword in a class/structure/interface definition)
+but they too can be explicitly typed or have their type inferred from being initialised in 
+the declaration. [[BR]]
+Like method parameter variables, if untyped and uninitialised, their type defaults to type '''dynamic'''.
+
+Instance variables default to public access. [[BR]] 
+If named with a leading '_' they default to protected access, with double leading '_' to private access. [[BR]]
+Accessibility may be explicitly specified otherwise or overridden with an 'is clause' specifying one 
+of 'public', 'protected', 'private'. 
+
+{{{
+class VarEG
+    var count = 0  # type inferred from initial value (int32)
+    var age as int # explicitly typed to int (int32) initial value default value for int (0)
+
+    var _name as String # explicitly typed to String - nil value, private 
+    var _definition as INamedNode?
+    
+    var other is private # dynamic type, explicitly private
+
+    # Properties    
+    pro defn as String  # String Property
+        get
+            if not _definition
+                return 'No Defn'
+            return _definition.name 
+        set
+            assert value.name   
+            _definition = value    
+
+    pro name from _name     # Read-write String Property from _name 
+
+    get repr  as String     # Read-only String property - synthesized
+        return '[_name]_[.age]'        
+
+    # class Variable
+    var nItems as int is shared
+
+}}}
+
+Unlike local variables, explicitly typed instance variables without an initial value given need  
+not be declared nilable unless they are intended or allowed to have a nil value after the 
+class instance initializer has run.[[BR]]
+Instance variables that are not nilable will emit a runtime error if they have a nil value 
+at the end of the initializer execution.
+
+In code, instance variables are accessed with a leading 'this.' or just '.'. [[BR]] 
+(idiomatically and preferably just a leading '.').[[BR]]
+The leading '.' is unnecessary if the instance variable name has a leading '_'. [[BR]]
+(These are unambiguously instance variables as local variables cannot be declared with 
+leading underscores.)
+
+{{{
+    # Initializer/Ctor
+    cue init( age as int, name as String)
+        .age = age 
+        _name = name    # ok _name now initialised and non nil
+        .other = .calcOther(name, age) 
+        .nItems += 1
+    
+    # A method
+    def rename(nuName as String) 
+        .count += 1  # instance variable
+        calcName = nuName + '_[.count]'   # local variable (String)
+        .name = calcName    # change instance variable
+}}}
+
+In the initializer, instance variable _name is now initialised, [[BR]]
+_definition is nil but thats allowed since its declared nilable.
+
+ 
+== Method parameter variables ==
+
+Methods (initializers, etc) in a class may be declared to take parameters. [[BR]] 
+These can be optionally explicitly typed. If method parameters are not explicitly typed they 
+are assumed to be the '''dynamic''' type.
+
+Method parameter variables are named the same way as local variables ( leading lowercase letter)
+and are also local only to the method they are on. [[BR]]
+Calls to the methods convey the args given in the call to the (formal) parameter names in the order 
+declared on the method for use in the called method.
+
+{{{
+
+    # call to create VarEg instance
+    t = VarEG( 157, 'hops')
+    #...
+    t.rename('hopscc')
+}}}
+
+In the initializer call parameter variables 
+    * age  <- 157 
+    * name <- 'hops'
+in method call rename, parameter variables
+    * nuName <- 'hopscc'
+
+== Properties ==
+
+Properties are a construct for external change or access to a classes state 
+(class or instance variables) mediated by some code. [[BR]]
+They can be a simple passthrough to a possibly otherwise inaccessible class variable (field) or 
+anything else doable in a method. 
+
+The major difference/advantage is that to calling code they are written to appear exactly like a 
+class variable access or mutation (no punctuation or boilerplate for method calls) so all the 
+description for instance variables applies.
+
+{{{
+
+    t = VarEG( 492, 'hops')
+    oldDefn = t.defn    # local var oldDefn <-  'No defn'
+    oldName = t.name    # local var oldName <- 'hops'
+    t.name ='hopscc'   
+
+    t.defn = .makeDefn(t)
+    print t.repr        # prints 'hopscc_492'    
+
+}}}
+
+== Class Variables ==
+
+Class (or shared or static) variables are like instance variables but they are associated with 
+the class rather than each instance of a class. 
+They are declared like instance variables except that they have the modifier 'shared'.
+Class variables can be accessed through a class instance or using the className
+otherwise all the description of instance variables applies.
+
+{{{
+    # Note that nItems is incremented in the initializer so it has a count of the 
+    # number of instances made
+    
+    nItems = VarEG.nItems
+    print 'number Of VarEg instances made ' + nItems
+    
+    nullVarEG = VarEg(0, '')
+    assert  nullVarEG.nItems == nItems + 1 # access class vars through instance or className
+}}}
+
+
+
+== Notes ==
+
+== See Also ==
+
+
+[wiki:LanguageTopics Back to LanguageTopics]