Praxis 101: Bonus calculation rule

This is supposed to be a gentle introduction to using Praxis. In this example we will perform a simple yearly bonus calculation for employees, based on their age and years of employment at the company

Torbjörn Josefsson
29 steps
10 minutes

Toblotron

Navigate to [https://toblotron.com/praxis/0.1.0/](https://toblotron.com/praxis/0.1.0/)

As soon as you get to the Praxis IDE you will see a new, empty model.

Let's give our model a name, and create a rules-page

To create our first rule, we drag out a Predicate shape from the palette Click here.

We'll name it "bonus", and fill in the first two arguments with "Age" and "Employed"

Select the field "Rule name", and type "bonus [[tab]] [[tab]] Age [[tab]] Employed"

Click "+" to add an argument - this will be the output argument, and we'll name it "Bonus" Click "Ok"

In Prolog (and Praxis), a word starting with an upper-case letter is a Variable, and these can hold Any value

Click on the background to de-select the shape, so we can see the Palette again

The new rule, "bonus/3" (/3 means it has 3 arguments) can already be used - but it doesn't do anything yet, so let's start adding Conditions that will decide if the rule succeeds, and in that case what happens.

Now we are going to work with Formula shapes, to make our decisions. Type "Employed [[tab]] < [[tab]] 5" Click "Ok"

Copy the shape, paste it, and move the copy where it should be Mark the shape/ shapes you want to copy Press [[ctrl]] + [[c]] Press [[ctrl]] + [[v]]

In this shape we want to allow for the case when Employed is More than 5 years, instead of less. Type ">" Click "Ok" Click outside shape to see Palette.

Drag out a Connection shape from the palette, and connect the shapes to it Connect by dragging from bottom-circles to top-circles.

Continue building the decision-tree, in the same fashion

Now we have completed an entire branch of our tree - if Employed > 5 and Age > 40, Bonus will be 100!

Now we must complete the other branch, which will contain more decisions

Ok - I just remembered I have to allow for the value 5 of Employed - as it is now, all values but 5 will allow the rule to proceed, but not 5 :)

Enter ">=" instead of ">" here, and we're done

Now we must assign a value to Bonus in each of the remaining branches - otherwise we won't have any answer in those cases Select the Formula shape, and click "+" to add a calculation-row Click this text field. Type "Bonus [[tab]] = [[tab]] 200" Click "Ok"

And the other one..

Click this text field.

Click "+"

Click this text field.

Write "Bonus[[TAB]]is[[TAB]]Age * 10" Click "Ok"

Here we used the "IS" operator, which is needed to make calculations. Writing (for example) "A = 1*2" will not put "3" in the variable "A" - it will contain "1*2" - so we need to Tell Prolog to actually Perform a calculation; not just describe it.

Now we are ready to use our rule! If your query-window contains something else, just click the "bonus/3" rule-shape to enter the query "bonus(Age,Employed,Bonus" Click "Query"

Ok - We got an error! That is because the rule only works as it should if there are Numbers in the variables Employed and Age - we are trying to make comparisons on them, after all!

Change the query to "bonus(11,22,Bonus)." to make a correct call Then click "Query" again

Ok! Now we got the result "Bonus = 200", which is what we wanted, right? Now enter "33" instead of "11", to say that the employee is 33 years

Click "Query"

PS - you see the button named "Reset"? This is a button you have to press when you have changed the model, and want to use the new changes - if you don't, Praxis will (at least in version 0.1.0) continue using the code/ logic from when you first pressed "Query" in a session. This is because we may want to remember things inside our Prolog session, and this is only reset when you press "Reset"

We got the same result, because Age is still less than 40

Enter "44" as the first argument, and click "Query"

Ok, now we got 440! That's Age (44) * 10 - just like we wanted!

And this is where we stop, for now In a followup installment to this tutorial we will look at more basic Praxis functionality using this example domain. We will have a short list of employees, with data concerning them, and show how we can use this data to make interesting decisions Stay tuned! :)