Changes between Version 2 and Version 3 of CoalesceExpression

Timestamp:

07/07/13 13:27:14 (11 years ago)

Author:

hopscc

Comment:

--

CoalesceExpression

@@ -1 @@
-= Coalesce Expression =
+= Coalesce Expressions =
+There are two forms of this - nil-coalesce and nonNil-coalesce. 
+In both cases they evaluate the first expression and if it is nil (nonNil) return the second expression otherwise return the first expression. 
 
-The '''coalesce''' binary expression evaluates to the first non-nil value. There is an augmented assignment version of it as well.
+They are convenient shorthand for respectively testing an expression against nil and returning a default or 
+testing against nonNil and returning something else (usually a deref of the first expression)
+
+= Nil Coalesce =
+ 
+The '''nil coalesce''' binary expression evaluates to the first non-nil value. There is an augmented assignment version of it as well.
 
 === Grammar ===
@@ -10 +17 @@
 The expression evaluates to a unless that value is '''nil''', in which case, it evaluates to '''b'''.
 
-Although uncommon, nothing prevents b itself from also being '''nil'''. Neither expression will be evaluated more than once and if a is non-nil then b will not be evaluated at all.
+Although uncommon, nothing prevents b itself from also being '''nil'''. 
+Neither expression will be evaluated more than once and if a is non-nil then b will not be evaluated at all.
 
 The type of the coalesce expression is the greatest common denominator between the type of '''a''' and the type of '''b'''.
@@ -19 +27 @@
 }}}
 
-In the augmented assignment version, the result is assigned back to a. This requires that b is type compatible with a or a compilation error will occur.
+In the augmented assignment version, the result is assigned back to a. 
+This requires that b is type compatible with a or a compilation error will occur.
 
+=== Examples ===
 {{{
 #!cobra
 # Example 1 
 print name ? 'NONAME' 
+
 
 # Example 2 
@@ -32 +43 @@
 
 # Example 3 
-get name as String
-    return _name ? .getType.name 
-
-# Example 4 
 # this: 
 name = if(employee.manager.name<>nil, employee.manager.name, 'NONAME') 
 # can evaluate the key expression twice and is less succinct than: 
 name = employee.manager.name ? 'NONAME' 
+
 }}}
+
+
+= !NonNil Coalesce =
+ 
+The '''nonNil coalesce''' binary expression evaluates the second expression if the first is a non-nil value.
+If the first expression is nil it just returns it. 
+There is an augmented assignment version of it as well.
+
+=== Grammar ===
+{{{
+<a> ! <b>
+}}}
+
+The expression evaluates to a if that value is '''nil''', otherwise it evaluates to '''b'''.
+
+Although uncommon, nothing prevents b itself from also being '''nil'''. 
+Neither expression will be evaluated more than once and if a is nil then b will not be evaluated at all.
+
+The type of the coalesce expression is the greatest common denominator between the type 'nil'  and the type of '''b'''.
+(Nilable Type of '''b''')
+
+=== Grammar ===
+{{{
+<a> != <b>
+}}}
+
+In the augmented assignment version, the result is assigned back to a. 
+This requires that b is type compatible with a or a compilation error will occur.
+
+Note: this looks like a boolean comparison (not equals) in other languages.
+
+{{{
+#!cobra
+# Example 1 
+print x ! x.name # if x is not nil return its member name else nil
+
+# name or default regardless of x or x.name nil
+print x ! x.name ? 'NONAME' 
+
+# Example 2 
+current = list
+current != current.head # (not a comparison)  nil if list nil or list.head nil
+if current
+    # process
+
+# Example 3 
+name = if(employee and employee.manager and employee.manager.name, employee.manager.name, 'NONAME') 
+# is less succinct than: 
+name = employee ! employee.manager ! employee.manager.name ? 'NONAME' 
+# assumes derefs are idempotent (same returns on each call or deref)
+
+# Example 4
+#if calls are NOT idempotent need to capture each deref and use it subsequently
+# assuming employee.manager returns nil or a different value on each visit as in some places
+mgrName = employee ! (m = employee.manager) ! m.name ? 'NONAME' 
+}}}