IF THEN ELSE using XML

Question

I have a program that uses XML formatted rules to create executable code for run-time  I have to define some actions and logical constructs using my own dialect  I have OR  AND  and NOT constructs and now I need to implement IF  THEN  ELSE   I m trying to come up with a syntax that would make sense and here s what I have so far    lt IF id  if-1  gt       lt TIME from  5pm  to  9pm    gt   lt  IF gt   lt THEN id  if-1  gt       lt    some actions defined     gt   lt  THEN gt   lt ELSE id  if-1  gt       lt    other set of actions defined here     gt   lt  ELSE gt    If looks difficult to read to me  but I don t see a cleaner way to represent this without doing too much nesting  Does anyone have a suggestion   not using XML is not an option at this point in time

User · Answer

I think that the thing you must keep in mind is that your XML is being processed by a machine, not a human, so it only needs to be readable for the machine.

In other words, I think you should use whatever XML schema you need to make parsing/processing the rules as efficient as possible at run time.

As far as your current schema goes, I think that the id attribute should be unique per element, so perhaps you should use a different attribute to capture the relationship among your IF, THEN, and ELSE elements.

User · Answer

I don t think you can design the if-then-else construct without taking the design for other constructs into account  I think it s a good principle that each expression should be an element  and its subexpressions should be child elements  There are then questions about whether the name of an element should reflect the type of expression it is  or its role relative to the parent  Or you can do both    lt if gt     lt condition gt       lt equals gt         lt number gt 2 lt  number gt         lt number gt 3 lt  number gt       lt equals gt     lt condition gt     lt then gt       lt string gt Mary lt  string gt     lt  then gt     lt else gt       lt concat gt         lt string gt John lt  string gt         lt string gt Smith lt  string gt       lt  concat gt     lt  else gt   lt  if gt    But you can sometimes get away with a design that omits the role-names  condition  then else  and relies on positional significance of elements relative to their parent  It depends a bit on how much you want to keep it concise

User · Answer

lt IF gt       lt CONDITIONS gt           lt CONDITION field  time  from  5pm  to  9pm  gt  lt  CONDITION gt       lt  CONDITIONS gt       lt RESULTS gt  lt    some actions defined     gt  lt  RESULTS gt       lt ELSE gt           lt RESULTS gt  lt    some other actions defined     gt  lt  RESULTS gt               lt  ELSE gt   lt  IF gt    Here s my take on it  This will allow you to have multiple conditions

User · Answer

lt IF id  if-1  gt      lt TIME from  5pm  to  9pm    gt   lt ELSE gt      lt something else   gt   lt  ELSE gt   lt  IF gt    I don t know if this makes any sense to anyone else or it is actually usable in your program  but I would do it like this   My point of view  You need to have everything related to your  IF  inside your IF-tag  otherwise you won t know what ELSE belongs to what IF  Secondly  I d skip the THEN tag because it always follows an IF

User · Answer

I personally think that the if then else needs to be linked somehow    lt IF something gt       lt some actions gt   lt THEN something gt       lt some actions gt   lt  THEN gt   lt ELSE something gt       lt some actions gt   lt  ELSE gt   lt  IF gt

User · Answer

Perhaps another way to code conditional constructs in XML    lt rule gt       lt if gt           lt conditions gt               lt condition var  something  operator   amp gt   gt 400 lt  condition gt               lt  -- more conditions possible -- gt           lt  conditions gt           lt statements gt               lt  -- do something -- gt           lt  statements gt       lt  if gt       lt elseif gt           lt conditions gt  lt  conditions gt           lt statements gt  lt  statements gt       lt  elseif gt       lt else gt           lt statements gt  lt  statements gt       lt  else gt   lt  rule gt

User · Answer

Faced with a similar problem sometime ago  I decided to go for a generalized  switch     case     break     default  type solution together with an arm-style instruction set with conditional execution  A custom interpreter using a nesting stack was used to parse these  programs   This solution completely avoids id s or labels  All my XML language elements or  instructions  support a  condition  attribute which if not present or if it evaluates to true then the element s instruction is executed  If there is an  exit  attribute evaluating to true and the condition is also true  then the following group of elements instructions at the same nesting level will neither be evaluated nor executed and the execution will continue with the next element instruction at the parent level  If there is no  exit  or it evaluates to false  then the program will proceed with the next element instruction  For example you can write this type of program  it will be useful to provide a noop  statement  and a mechanism instruction to assign values and or expressions to  variables  will prove very handy     lt ins-1 gt       lt ins-11 condition  expr-a  exit  true  gt           lt ins-111   gt                   lt  ins11 gt       lt ins-12 condition  expr-b  exit  true    gt       lt ins-13 condition  expr-c    gt       lt ins-14 gt                   lt  ins14 gt   lt  ins-1 gt   lt ins-2 gt           lt  ins-2 gt    If expr-a is true then the execution sequence will be   ins-1 ins-11 ins-111 ins-2   if expr-a is false and expr-b is true then it will be   ins-1 ins-12 ins-2   If both expr-a and expr-b are false then we ll have   ins-1 ins-13  only if expr-c evaluates to true  ins-14 ins-2   PS  I used  exit  instead of  break  because I used  break  to implement  breakpoints   Such programs are very hard to debug without some kind of breakpointing tracing mechanism   PS2  Because I had similar date-time conditions as your example along with the other types of conditions  I also implemented two special attributes   from  and  until   that also had to evaluate to true if present  just like  condition   and which used special fast date-time checking logic

User · Answer

Personally  I would prefer   lt IF gt     lt TIME from  5pm  to  9pm    gt     lt THEN gt       lt  -- action -- gt     lt  THEN gt     lt ELSE gt       lt  -- action -- gt     lt  ELSE gt   lt  IF gt    In this way you don t need an id attribute to tie together the IF  THEN  ELSE tags

User · Answer

lt IF id  if-1  gt       lt CONDITION gt           lt TIME from  5pm  to  9pm    gt       lt  CONDITION gt       lt THEN gt           lt    some actions defined     gt       lt  THEN gt       lt ELSE gt           lt    other set of actions defined here     gt       lt  ELSE gt   lt  IF gt    Seems easier to read to me  There s more nesting  but if anything that helps with the readability

[xml] IF...THEN...ELSE using XML

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