Adding conditional steps to config prompt

[henriquemoraeszup]

Description

As requested on issue #390, this PR allows the developer to specify optional inputs according to previous user inputs. The user needs to create a key called condition on an optional step and provide a variable reference from any previous variable defined, an operator for comparison (==, !=, >, >=, <, <=), and a value to compare. If the condition exists and fails, the current step is not shown.

This is a simple implementation some of the points taken in account are explained below:

• The comparison operators are implemented in a case like manner. It is not very elegant. The better approach would be to evaluate the expression, but it was not chosen because:
  1. Go does not come with a native eval function and it would require to add a dependency to the project for a simple task
  2. Since many times the user input is free we could risk him adding harmful code as a value and executing it on an eval command or letting the author add malicious code on the operator variable
• This approach only attends simple expressions between a single variable and a value. Complex multi-variable expressions are not supported for the sake of avoiding overengineering. This should cover most of the cases, I think it is better to wait for the community to demand more complex interactions before we actually implement them.
• There are discussions to create tree-like decisions to prompt the user. This implementation does not rule that out, we can still implement a decision tree with conditional nodes, so forward-compatibility is accounted for.
• In case the user uses a wrong operator. An error is thrown
• In case the user defines a non existing variable name. An error is thrown

Suggestions are welcome!

How to verify it

Create a condition key on config.json of a formula like on this example

{
      "cache": {
        "active": true,
        "newLabel": "Type new value. ",
        "qty": 6
      },
      "label": "Type : ",
      "name": "sample_text",
      "condition": {
          "variable": "sample_list",
          "operator": "==",
          "value": "in_list1"
      },
      "type": "text"
    },

Run the formula and fail the condition to have this prompt not shown

Description for the changelog

Adding conditional steps to config prompt

[codecov-commenter]

Codecov Report

Merging #454 into master will increase coverage by 0.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
+ Coverage   66.75%   67.20%   +0.45%     
==========================================
  Files          80       80              
  Lines        2755     2784      +29     
==========================================
+ Hits         1839     1871      +32     
+ Misses        735      733       -2     
+ Partials      181      180       -1     

Impacted Files	Coverage Δ
pkg/formula/formula.go	90.90% <ø> (ø)
pkg/formula/runner/inputs.go	98.74% <100.00%> (+2.58%)	⬆️

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[kaduartur]

Good job 🥇

[rasouza]

Amazing job indeed 👏

One further improvement suggestion: we could implement a parser to get rid of the condition object and let it be written as simples as

...
"condition": "sample_list == in_list1"
...

This implementation of yours is great already, though. Thanks!

[kaduartur]

/merge qa

[ritchie-bot]

👌 Merged branch feature/conditional-variables into qa

[henriquemoraeszup]

Actually the parser is a good idea, it would allow us to cover the point above on multi-variables as well. We can leave as an improvement =]

Hashicorp's bparser could take us a long way
https://github.com/hashicorp/go-bexpr

Amazing job indeed 👏

One further improvement suggestion: we could implement a parser to get rid of the condition object and let it be written as simples as

...
"condition": "sample_list == in_list1"
...

This implementation of yours is great already, though. Thanks!