[Date Index]
[Thread Index]
[Author Index]
LocalRule & Local Set operators
*To*: mathgroup at smc.vnet.net
*Subject*: [mg13908] LocalRule & Local Set operators
*From*: "Ersek, Ted R" <ErsekTR at navair.navy.mil>
*Date*: Sun, 6 Sep 1998 02:55:44 -0400
*Sender*: owner-wri-mathgroup at wolfram.com
Recall the messages I sent on 28 Aug and 29 Aug. [mg13812] An operator
called-- LocalRule [mg13835] An operator called-- LocalSet
I have improved versions of these functions below (with usage messages).
The differences are:
(1) The new versions will not look inside Verbatim[....] to find
patterns that should be evaluated inside a local environment.
(2) The new versions use a new function PatternNames which may be
useful in it's own right. The program for PatternNames is a bit
advanced, but it does with one application of Cases what took a few
lines in my previous version.
(3) This version will not have trouble if the same pattern name appears
in (lhs) more than once.
(e.g. f[x_, x_] )
Changes (1) and (2) are due to feedback I got from one of the gurus in
the group. I understand there are still some subtle problems with
these programs, but they should be useful for many applications as they
are.
Cheers,
Ted Ersek
(*********************************)
Unprotect[LocalRule, RightArrow, LocalSet, DotEqual, PatternNames];
LocalRule::usage=
"LocalRule[lhs,rhs] represents a rule where rhs is evaluated \
immediately using a local environment for any variable used as a \
pattern in lhs. LocalRule[lhs,rhs] is equivalent to \
lhs\[RightArrow]rhs where \[RightArrow] is entered \ as
\\[RightArrow].";
RightArrow::usage=
"lhs\[RightArrow]rhs represents a rule where rhs is evaluated \
immediately using a local environment for any variables used as a\
pattern in lhs. Note: \[RightArrow] is entered as \\[RightArrow].\
The expression lhs\[RightArrow]rhs is equivalent to \
LocalRule[lhs,rhs].";
LocalSet::usage=
"LocalSet[lhs,rhs] evaluates rhs using a local environment for any \
variable used as a pattern in lhs. From then on lhs is replaced by the
\ result of evaluating rhs whenever lhs appears. LocalSet[lhs,rhs] is
\ equivalent to lhs\[DotEqual]rhs where \[DotEqual] is entered \ as
\\[DotEqual].";
DotEqual::usage=
"lhs\[DotEqual]rhs evaluates rhs using a local environment for any \
variable used as a pattern in lhs. From then on lhs is replaced by \
the result of evaluating rhs whenever lhs appears. The expression \
lhs\[DotEqual]rhs, is equivalent to LocalSet[lhs,rhs].";
PatternNames::usage=
"PatternNames[expr] returns a list of the names of patterns \
included in expr. The list returned is wrapped in Hold. \ PatternNames
does not consider patterns that appear inside Verbatim. \ Example:\n
PatternNames[foo[a_,b_:1,c_?NumericQ,d,e]] returns \n
Hold[{a,b,c}].";
LocalSet=DotEqual;
LocalRule=RightArrow;
Attributes[DotEqual]={HoldAll,SequenceHold};
Attributes[RightArrow]={HoldRest,SequenceHold};
Attributes[PatternNames] = {HoldFirst};
PatternNames[lhs__] :=
Module[{lhsWithoutVerbatim, heldNames},
lhsWithoutVerbatim = ReplaceAll[Hold[lhs], _Verbatim -> Null];
heldNames = Cases[lhsWithoutVerbatim,
Verbatim[Pattern][name_Symbol,_] :> Hold[name],
{0,-2}, Heads -> True
];
Thread[Union at heldNames, Hold]
]
RightArrow[lhs_,rhs_]:=Module[{q},
q=PatternNames[lhs];
If[q==={},(lhs:>Evaluate at rhs),
Block@@Join[q,Hold[lhs:>Evaluate at rhs] ]
]]
DotEqual[lhs_,rhs_]:=Module[{b1,b2},(
b1=PatternNames[lhs];
If[b1==={},(Unevaluated at lhs:=Evaluate at rhs),
b2=Hold[#:=Evaluate at rhs]& @@{Unevaluated at lhs};
Block@@Join[b1,b2]
]
)]
Protect[LocalRule, RightArrow, LocalSet, DotEqual, PatternNames];
Prev by Date:
**Help!!!!**
Next by Date:
**Re: Postscript Error**
Previous by thread:
**Help!!!!**
Next by thread:
**Macintosh Print Anomaly**
| |