DefineExpr

From Erights

(Difference between revisions)
Jump to: navigation, search
(→Examples)
(specify DefineExpr)
Line 1: Line 1:
[[DefineExpr]] is a [[Kernel-E]] node which matches a [[pattern]] against a specified value.
[[DefineExpr]] is a [[Kernel-E]] node which matches a [[pattern]] against a specified value.
-
"def" <var>pattern</var> ("exit" <var>optExitExpr</var>)? ":=" <var>specimen</var>
+
"def" <var>pattern</var> ("exit" <var>optEjectorExpr</var>)? ":=" <var>rValue</var>
Note that the syntax is identical to [[DefrecExpr]], the nonkernel expression which allows ''recursive'' definitions; DefineExpr rejects such uses.
Note that the syntax is identical to [[DefrecExpr]], the nonkernel expression which allows ''recursive'' definitions; DefineExpr rejects such uses.
Line 8: Line 8:
* <var>pattern</var> :[[Pattern]]
* <var>pattern</var> :[[Pattern]]
-
* <var>optExitExpr</var> :[[nullOk]]<nowiki>[</nowiki>[[EExpr]]]
+
* <var>optEjectorExpr</var> :[[nullOk]]<nowiki>[</nowiki>[[EExpr]]]
-
* <var>specimen</var> :[[EExpr]]
+
* <var>rValue</var> :[[EExpr]]
== Scope ==
== Scope ==
-
[[Node scope rules|seq]](<var>pattern</var>, <var>optExitExpr</var>, <var>specimen</var>)
+
[[Node scope rules|seq]](<var>pattern</var>, <var>optEjectorExpr</var>, <var>rValue</var>)
-
 
+
-
Note that this order differs from the evaluation order; the static constraints make this possible without [[promise]]s.
+
== Evaluation ==
== Evaluation ==
-
# If present, <var>optExitExpr</var> is evaluated.
+
# If present, <var>optEjectorExpr</var> is evaluated.
-
# <var>specimen</var> is evaluated.
+
# <var>rValue</var> is evaluated.
-
#<var>pattern</var> is matched against the result of <var>specimen</var>, with the result of <var>optExitExpr</var>, or [[throw]] if not present, as the ejector.
+
#<var>pattern</var> is matched against the result of <var>rValue</var>, with the result of <var>optEjectorExpr</var>, or [[throw]] if not present, as the ejector.
== Static constraints ==
== Static constraints ==
-
* <var>specimen</var> may not use nouns bound by <var>pattern</var>. (See [[DefrecExpr]].)
+
* <var>rValue</var> may not use nouns bound by <var>pattern</var>. (See [[DefrecExpr]].)
-
* <var>optExitExpr</var> may not use nouns bound by <var>pattern</var>.
+
* <var>optEjectorExpr</var> may not use nouns bound by <var>pattern</var>.
-
* <var>pattern</var> may not use nouns bound by <var>specimen</var>.
+
* <var>pattern</var> may not use nouns bound by <var>rValue</var>.
-
* <var>pattern</var> may not use nouns bound by <var>optExitExpr</var>.
+
* <var>pattern</var> may not use nouns bound by <var>optEjectorExpr</var>.
-
 
+
-
== Examples ==
+
-
 
+
-
Simple uses with [[FinalPattern]], [[VarPattern]] and [[LiteralExpr]]:
+
-
 
+
-
? def pi := 3.14159
+
-
# value: 3.14159
+
-
+
-
? pi
+
-
# value: 3.14159
+
-
 
+
-
? def var x := 1
+
-
> x
+
-
# value: 1
+
-
+
-
? x := 2
+
-
> x
+
-
# value: 2
+
-
 
+
-
With optExitExpr specified, but making no difference:
+
-
 
+
-
? def x :int exit throw := 9
+
-
# value: 9
+
-
+
-
? x
+
-
# value: 9
+
-
+
-
? def x :String exit throw := 9
+
-
# problem: the int 9 doesn't coerce to a String
+
-
 
+
-
With optExitExpr different:
+
-
 
+
-
? def x :int exit def _(e) :void { print(e); throw("biff") } := 9
+
-
# value: 9
+
-
 
+
-
? def x :String exit def _(e) :void { print(e); throw("biff") } := 9
+
-
# stdout: problem: the int 9 doesn't coerce to a String
+
-
+
-
# problem: biff
+
-
+
-
Demonstration of the evaluation order:
+
-
 
+
-
? def [[ViaPattern|via]] (fn s,_ {println("patt"); throw("exit")}) x \
+
-
>  exit (println("ejector"); null) \
+
-
>  := (println("specimen"); 8)
+
-
# stdout: ejector
+
-
#        specimen
+
-
#        patt
+
-
#       
+
-
+
-
# problem: exit
+
-
+
-
A bogus exit, which does not return when it should (this is [[throw.eject]] behavior):
+
-
 
+
-
XXX in [[E-on-Java]] this creates a note in the trace that the ejector returned and proceeds as if the ejector was null; the shown behavior is from [[E-on-CL]]; which to pick?
+
-
 
+
-
? def x :String exit def _(_) :void {} := 9
+
-
# problem: optEjector <__main$_> returned: null
+
-
+
-
Scope tests:
+
-
 
+
-
? {def y := 0; {def ==0 exit (def y := null) := y}}
+
-
# problem: null is not 0
+
-
 
+
-
XXX write more scope tests
+
[[Category:E language specification]]
[[Category:E language specification]]
[[Category:Kernel-E nodes]]
[[Category:Kernel-E nodes]]

Revision as of 05:03, 4 January 2009

DefineExpr is a Kernel-E node which matches a pattern against a specified value.

"def" pattern ("exit" optEjectorExpr)? ":=" rValue

Note that the syntax is identical to DefrecExpr, the nonkernel expression which allows recursive definitions; DefineExpr rejects such uses.

Contents

Fields

Scope

seq(pattern, optEjectorExpr, rValue)

Evaluation

  1. If present, optEjectorExpr is evaluated.
  2. rValue is evaluated.
  3. pattern is matched against the result of rValue, with the result of optEjectorExpr, or throw if not present, as the ejector.

Static constraints

  • rValue may not use nouns bound by pattern. (See DefrecExpr.)
  • optEjectorExpr may not use nouns bound by pattern.
  • pattern may not use nouns bound by rValue.
  • pattern may not use nouns bound by optEjectorExpr.
Retrieved from "http://wiki.erights.org/wiki/DefineExpr"
Personal tools
more tools