Changes between Initial Version and Version 1 of NilableTypes

Timestamp:

05/03/08 13:52:55 (17 years ago)

Author:

hopscc

Comment:

--

NilableTypes

@@ +1 @@
+= Nilable Types =
+
+Cobra forces specification of variables that may be allowed to contain '''nil'''.
+This is done by typing the variable as a nilable Type which is any typeName with a '?' suffix
+
+e.g.
+{{{
+    str as String = "Mystring"    # str can only have a String value
+    strN as String?               # strN may have a String value OR nil
+
+    lazyType as Type?             # Type OR nil
+}}}
+
+nilable Typing can be applied to anything that can be Typed ( variables, Args, parameters, returnTypes)
+
+{{{
+    def sendWelcome(cust as Customer, referral as Customer?) 
+        # will not accept nil for `cust`
+        # will accept nil for `referral` 
+}}}
+
+== Casting to and From nilable Types ==
+
+Sometimes it is necessary to cast an existing typed variable to a nilable type [[BR]]
+( e.g for receiving results of a library call) or [[BR]]
+to cast a nilable type to its non nilable type (when you've ascertained its not nil and the receiver only takes a non nilable type)
+
+There is a shorthand for both of these
+
+Casting to a nilable version of an existing type
+    <variable> to ?
+Casting away from nilable version
+    <variable> to !
+e.g.
+{{{
+    s as String = 'a string'
+    sn = s to ?    # sn is String?
+    assert sn
+    s1 = sn to !   # s1 is String - may not get nil
+}}}     
+
+== nil and not nil detection and coalesce ==
+
+You can explicitly test for nil or not nil values using normal conditionals ( true is non-nil)
+{{{
+    s as String = 'sss'
+    assert s
+    if s
+        print 's not nil'
+    sn as String? = nil
+    assert not sn
+    if not sn
+       print 'sn is nil'
+}}}
+
+Nil test and assign or non nil test and process can be done using 'nil coalescing'[[BR]]
+
+'''?''' coalesce if nil.[[BR]]
+
+'''! ''' execute if not nil.  
+{{{
+    stuff as String? = nil
+    defaultStuff='DEFAULT'
+ 
+    stuff = stuff ? defaultStuff  # coalesce nil
+    # equivalent to  stuff = if (not stuff, defaultStuff, stuff)
+    # or 
+    # if not stuff
+    #    stuff = defaultStuff
+    assert stuff == defaultStuff
+
+                   
+    def doSomething(input as String)
+        if input  # detect not nil
+            print 'Received some input.'
+
+        i as int? = input ! .parse(input)  # execute non-nil
+        # equiv to 
+        #    i = if(input, .parse(input), input)
+
+        # or alternatively
+        # input = input ? '0'
+        # i as int = .parse(input)
+
+}}}
+
+
+