Correct syntax to reference an azure pipeline variable inside an expression

I have a block like the following, where foo is a pipeline variable created in AzureDevOps and I want to access the value in my yaml file

variables:
  - name: MyVar
    ${{ if and(eq(variables['foo'], 'bar') , eq(parameters.environment, 'xyz')) }}:
     value: abc
    ${{ if and(eq(variables['foo'], 'baz') , eq(parameters.environment, 'pqr')) }}:
     value: def

the above doesn't seem to be the correct syntax though, as my variable MyVar remains empty. What is the correct syntax? Apart from the one above, I have tried the following without success

variables:
  - name: MyVar
    ${{ if and(eq(variables.foo, 'bar') , eq(parameters.environment, 'xyz')) }}:
     value: abc
    ${{ if and(eq(variables.foo, 'baz') , eq(parameters.environment, 'pqr')) }}:
     value: def

and

variables:
  - name: MyVar
    ${{ if and(eq($(foo), 'bar') , eq(parameters.environment, 'xyz')) }}:
     value: abc
    ${{ if and(eq($(foo), 'baz') , eq(parameters.environment, 'pqr')) }}:
     value: def

As Daniel wrote you are mixing compile-time expressions with run-time expressions.

This is possible to use variables in compile-time expression which is ${{ }}, but it has to be static variable.

variables:
  staticVar: 'my value' # static variable
  compileVar: ${{ variables.staticVar }} # compile time expression
  isMain: $[eq(variables['Build.SourceBranch'], 'refs/heads/main')] # runtime expression

steps:
  - script: |
      echo ${{variables.staticVar}} # outputs my value
      echo $(compileVar) # outputs my value
      echo $(isMain) # outputs True      

Please take a look on template expression doc

> Use template expressions to specify how values are dynamically resolved during pipeline initialization. Wrap your template expression inside this syntax: ${{ }}.
>
> Template expressions can expand template parameters, and also variables. You can use parameters to influence how a template is expanded. The parameters object works like the variables object in an expression. Only predefined variables can be used in template expressions.

It could be somehow misleading with runtime parameters which suggest belonging to run-time evaluation:

> Parameters are only available at template parsing time. Parameters are expanded just before the pipeline runs so that values surrounded by ${{ }} are replaced with parameter values. Use variables if you need your values to be more widely available during your pipeline run.