Casting to float: document how to do it

[manucarrace]

In order to make a casting to float it is needed to make the following
(cast(cast(battery?, String), float)

If we make (cast(battery?, float) the rule does not work as expected.

[AlvaroVega]

In (cast(battery?, float) is there a missed ) ?

[fgalan]

Is this actually a problem with Perseo? Or just a consequence of how EPL works?

[AlvaroVega]

Could you provide a full example (rule, action, data, expected data) ?

In general if you use version 2 of NGSI for updateAction then float cast is done automatically:
https://github.com/telefonicaid/perseo-fe/blob/master/lib/models/updateAction.js#L280

[AlvaroVega]

if there is a temperature attribute of type Number, or inserted in ContextBroker as a real numerical type then you don't need to cast to string, just cast to float:

select *, *, ev.temperature? as Temperature,  ev.id? as Meter from pattern [every ev=iotEvent(cast(temperature?,float)>1.5)]

[mrutid]

Two nested cast expression are needed if you expect to receive Strings representing Numbers is your context stream.

Examples and explanation:

Every Dinamic Property (fillingLevel?) has Object type in Esper, therefore casting is needed to use fillingLevel in float expressions.

If we do the float cast it works when fillingLevel is a number (int or float). But fails if it's a String containing a number (in fact an Object containing a String "containing" a number):

If we resolve the Object to String (which works almost every time) then we can safety cast to float:

In this case if filling level contains a value uncasteable to float ("PATATA") an exception occurs, so rule doesn't get fired.

[AlvaroVega]

It would be nice document that explanation #345 (comment) into doc about EPL usage https://github.com/telefonicaid/perseo-fe/blob/master/docs/API/plain_rules.md#epl-text