How to set "text-font" with mapbox expressioins

I am trying to set the font with the help of Mapbox expressions from font_size field. This doesn't work for me (Invalid data expression for "text-font". Output values must be contained as literals within the expression)

 "text-font":  [
      "case",
      ['!=', ["get", "text_font"], ""],
      ["get", "text_font"],
      ['literal',['DIN Offc Pro Italic', 'Arial Unicode MS Regular']]
  ],

So I want to get a text-font from text_font: ["Open Sans Semibold Italic"] and if it is empty like text_font: "" use ['DIN Offc Pro Italic', 'Arial Unicode MS Regular'] Thanks

I think the error message is telling you what you need to know, but don't want to hear:

Invalid data expression for "text-font". Output values must be contained as literals within the expression.

The output values (that is, the font that is going to be set) must be specified as a literal - not derived from a feature property.

So you could do something like this as a workaround:

"text-font":  [
  "match", ["get", "text_font"],
    "Open Sans Semibold Italic", ["literal", ["Open Sans Semibold Italic"]],
    "Arial", ["literal", ["Arial"]],
    // ... all possible values ...
    ['literal',['DIN Offc Pro Italic', 'Arial Unicode MS Regular']]
  ]
],