`cond` blocks in template problem

[silverdr]

When trying to write cond do block inside a template I get

** (SyntaxError) lib/phxapp_web/templates/entity/index.html.slim:1:2380: syntax error before: '->'
    |
  1 |  PhxApp.function_returning_bool?(argument) -> {"string0", "string1"}
    |                                            ^

Am I doing something wrong or is this just a no-go?