[REBOL] Re: a new switch-like flow-control function
From: joel:neely:fedex at: 17-Dec-2003 10:19
Hi, Anton,
Set the WayBack machine to 2000... ;-)
Anton Rolls wrote:
> I yearn (want), occasionally, a kind of any/switch-like
> flow-control function...
Some years ago we had a long discussion about generalizing IF on the
list, and kicked around various options/versions. The copy I could
lay my hands on most quickly was Ladislav's refinement (the last in
the discussion???):
pif: func [[throw catch] args [block!] /local res] [
either unset? first res: do/next args [
if not empty? args [
throw make error! [script no-arg pif condition]
]
][
either first res [
either block? first res: do/next second res [
do first res
][
throw make error! [
script expect-arg pif block [block!]
]
]
][
pif second do/next second res
]
]
]
PIF (Polymorphic IF) takes a block of guard-expression/block
pairs and uses the (first true) guard-expressions to select
block to be evaluated. It can be used (I believe) essentially
as you were asking.
Refactoring PIF to iterative form is left as an exercise for
the reader! ;-) As long as the argument block isn't too long
the recursion likely won't be a problem.
Heres a sample usage:
describe: func [x [integer!] y [integer!]] [
pif [
x = 1 ["X is one"]
y = 1 ["Y is one"]
x = 2 ["X is two"]
y = 2 ["Y is two"]
true ["X and Y are too big!"]
]
]
which behaves as:
>> describe 1 1
== "X is one"
>> describe 2 1
== "Y is one"
>> describe 2 2
== "X is two"
>> describe 3 2
== "Y is two"
>> describe 3 3
== "X and Y are too big!"
-jn-
--
----------------------------------------------------------------------
Joel Neely joelDOTneelyATfedexDOTcom 901-263-4446
Enron Accountingg in a Nutshell: 1c=$0.01=($0.10)**2=(10c)**2=100c=$1
