[Proposal] MathExpressionConverter boolean support

[stephenquan]

This is a Math Expression Converters #71 refactor enhancement to improve the parser by (1) using a Parsing expression grammar, and (2) to support boolean operations. The grammar is now:

Expr        ::= Conditional
Conditional ::= LogicalOr ( '?' LogicalOr ':' LogicalOr ) ?
LogicalOr   ::= LogicalAnd ( '||' LogicalAnd ) *
LogicalAnd  ::= Equality ( '&&' Equality ) *
Equality    ::= Compare ( ( '==' | '!=' ) Compare ) *
Compare     ::= Sum ( ( '>=' | '>' | '<=' | '<' ) Sum ) *
Sum         ::= Product ( ( '+' | '-' ) Product ) *
Product     ::= Power ( ( '*' | '/' ) Power ) *
Power       ::= Primary ( '^' Primary ) *
Primary     ::= Number
              | Function
              | Constant
              | '(' Expr ')'
              | ( '-' | '!' ) Primary
Number      ::= ( '-' ) ? ( ( [0-9]+ '.' [0-9]+ ) | [0-9]+ )
Function    ::= [a-z]+ ( '(' ')' | '(' Expr ( ',' Expr ) * ')' )
Constant    ::= [a-z]+

Where Conditional, LogicalOr, LogicalAnd, Equality, and Compare are new. true and false boolean constants are also new. Sum, Product, Power, Primary, Number, Function, and Constant are a refactor of what was already supported by the original parser.

The implementation can be found:

• in my fork: https://github.com/stephenquan/Maui
• preview change commits: main...stephenquan:Maui:main

References:

• https://en.wikipedia.org/wiki/Parsing_expression_grammar
• https://en.wikipedia.org/wiki/Order_of_operations#Programming_languages

[stephenquan]

For instance, if you wanted to implement the conditional Label.TextColor = x0 + x1 + x2 >= 60 ? Colors.Green : Colors.Red you can make use of the MultiMathExpression and write it as follows:

<Label.TextColor>
    <MultiBinding Converter="{StaticResource MultiMathExpressionConverter}" ConverterParameter="x0 + x1 + x2 &gt;= 60 ? x3 : x4">
        <Binding Mode="OneWay" Path="X0" />
        <Binding Mode="OneWay" Path="X1" />
        <Binding Mode="OneWay" Path="X2" />
        <Binding Source="{x:Static Colors.Green}" />
        <Binding Source="{x:Static Colors.Red}" />
    </MultiBinding>
</Label.TextColor>

Alternatively, you can solve the same problem using DataTrigger by having Green as the default color and as Red as the non-default. This pattern has merit when we want to define multiple setters for the same trigger.

<Label
    Grid.Row="1"
    Grid.Column="7"
    Text="&gt;= 60"
    TextColor="Green"
    VerticalOptions="Center"
    VerticalTextAlignment="Center">
    <Label.Triggers>
        <DataTrigger TargetType="Label">
            <DataTrigger.Binding>
                <MultiBinding Converter="{StaticResource MultiMathExpressionConverter}" ConverterParameter="x0 + x1 + x2 &gt;= 60">
                    <Binding Mode="OneWay" Path="X0" />
                    <Binding Mode="OneWay" Path="X1" />
                    <Binding Mode="OneWay" Path="X2" />
                </MultiBinding>
            </DataTrigger.Binding>
            <DataTrigger.Value><x:Boolean>False</x:Boolean></DataTrigger.Value>
            <Setter Property="Label.TextColor" Value="Red" />
        </DataTrigger>
    </Label.Triggers>
</Label>

[CliffAgius]

I like this idea and it's got some community support I have tagged to be discussed at this months meetup on Thursday 5th September if your able to attend you could add to the conversation.

[stephenquan]

@CliffAgius, thanks, I will try to make it.

[CliffAgius]

Thanks for attending the meetup you can read about contributing to the MCT Here.

So that you have an issue to create a PR against I have converted this discussion into an Issue #2181 for you.

[CliffAgius]

This discussion was approved in the September monthly stand-up and approved by the community so has been converted into Issue #2181 for the work to begin.

Therefore I am closing this discussion as all work will continue on the Issue for tracking.