support if statement

[lixingwang]

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[] bug report => search github for a similar issue or PR before submitting
[*] enhancement request
[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
Today we only support ternary expression for one level if else case, it has the limitation and it does not support nested ternary expression.
Expected behavior:
We should come up with an if statement expression which supports nested if else. such as:

 if true {
  "yes" 
 }else {
   "no"
}

if string.equals("FLOGO", "FLOGO") {
     if string.equals("if", "if") {
             "Hello FLOGO IF expression" 
     } 
}else {
    if string.equals("TIBCO", "TIBCO") {
            "Hello TIBCO"
    }
}

The if statement gives the user more flexibility to achieve more complex use cases

Minimal steps to reproduce the problem (not required if feature enhancement):

What is the motivation / use case for changing the behavior?

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X

Additional information you deem important (e.g. issue happens only occasionally):

[lixingwang]

There is a use case that query salesforce approval object and return the status of the record with description.
Here is the status and expect return messages:

Submitted  ->  Your request has been submitted.
Pending	->  Your request staus still pending.
Approved  -> Your request has been approved.
Rejected	-> Your request has been rejected
REassgined  -> Your request has been submitted for approval but assigned to a different approver.
Recalled	->  You request has been recalled.

Possible solution

1. User want to have only one return so branching not work.(we don't have branch merge today)
2. Ternary expression, we don‘t support nested ternary expression so far. it might hard to read if we support nested. such as:

$activity[query].output.Approval == ”Submitted“ ? ”Your request has been submitted“ : ($activity[query].output.Approval == ”Pending“ ? "Your request staus still pending" : ($activity[query].output.Approval == "Approved" ? "Your request has been approved": ($activity[query].output.Approval == "Rejected" ? "Your request has been Rejected": ($activity[query].output.Approval == "Reassgined" ? "Your request has been reassigned":($activity[query].output.Approval == "Recalled" ? "Your request has been Recalled":"Unknow staus"))))）

3. If statment

	if $activity[query].output.Approval == ”Submitted“  {
			”Your request have been submitted“
	}else if $activity[query].output.Approval == "Pending" {
			"Your request staus still pending."
	}else if $activity[query].output.Approval == "Approved" {
			"Your request has been approved."
	}else if $activity[query].output.Approval == "Rejected" {
			"Your request has been rejected"
	}else if $activity[query].output.Approval == "Reassgined" {
			"Your request  has been submitted for approval but assigned to a different approver"
	}else if $activity[query].output.Approval == "Recalled" { 
			"You request has been recalled."
	}else {
			"Unknow staus"
	}

We can see the if/else statement much simple and clear for this kind of usecase.
@mellistibco @fm-tibco Please let me know your thougts? or possible others resolution?

[lixingwang]

@mellistibco @fm-tibco please add your comments and suggestions.

[mellistibco]

thanks @lixingwang ... I just still think I'd rather have a scripting activity that would support something like this rather than supporting it via the expression syntax. I really think expressions should be lightweight expressions and should not begin to morph into a scripting language...

[lixingwang]

Thanks, @mellistibco, it makes sense. I thinking about to add an activity to support this. BTW, I created #263 to support nested ternary expression.