why doesn't string variable containing @media work?

[719media]

why doesn't this work?

$test:  '@media (max-width: 699px)';

.thingamabob {
  #{$test} {
    color: yellow;
  }
}

but a normal selector does:

$test:  '.some-class';

.thingamabob {
  #{$test} {
    color: yellow;
  }
}

[nex3]

This is due to the order in which Sass parses different syntactic forms. When the parser first parses the document, it decides what each rule is—whether it's a style rule, a media directive, or something else entirely. Interpolation, like the #{$test} in your example, is classified as a style rule. Then when it executes the stylesheet, it tries to parse whatever the contents of the rule are as a selector. @media (max-width: 699px) isn't a valid selector, so this parsing fails.

If you're trying to dynamically choose between a selector or a media query, I suggest using a mixing with a @content directive.

[719media]

Thanks for the documentation pointer, I'll read it over. The problem is that I wanted a single user defined variable to be used, and I wanted the user to be able to use either
$hover-setting: '.custom-selector';
or
$hover-setting: '@media (hover: hover);

for use in a mixin like

@mixin hover {
  #{$hover-setting} {
    &:hover { @content; }
  }
}

but, it appears I'll have to use just two variables, and do something like

@mixin hover {
  @if $hover-selector {
    #{$hover-selector} {
      &:hover { @content; }
    }
  } @else if $hover-media {
    @media #{$hover-media} {
      &:hover { @content; }
    }
  } @else {
    &:hover { @content; }
  }
}