Why are parentheses not compiling after #{$var} in SASS?

Question

I have the following @mixin in SASS:

@mixin complexTransform($axis, $perspective, $degrees, $axis2, $px) {

    -webkit-transform: 
        perspective( $perspective )
        rotate#{ $axis }( $degrees )
        translate#{ $axis2 }( $px );

    -moz-transform: 
        perspective( $perspective )
        rotate#{ $axis }( $degrees )
        translate#{ $axis2 }( $px );

    -o-transform: 
        perspective( $perspective )
        rotate#{ $axis }( $degrees )
        translate#{ $axis2 }( $px );

    -ms-transform:
        perspective( $perspective )
        rotate#{ $axis }( $degrees )
        translate#{ $axis2 }( $px );

    transform:
        perspective( $perspective )
        rotate#{ $axis }( $degrees )
        translate#{ $axis2 }( $px );
}

Everything compiles nicely, except the parentheses after rotate#{$axis} and translate#{$axis}. This is what the compiled CSS looks like:

  -webkit-transform: perspective(600px) rotateX45deg translateY75px;
  -moz-transform: perspective(600px) rotateX45deg translateY75px;
  -o-transform: perspective(600px) rotateX45deg translateY75px;
  -ms-transform: perspective(600px) rotateX45deg translateY75px;
  transform: perspective(600px) rotateX45deg translateY75px;

What is it that I am doing wrong? Or is this a completely wrong way to go about doing this?

Thank you in advance!

EIDT: Nevermind! Found a solution... it might be a bit 'hacky' but at least it works. I am posting it here in case anyone else encounters the same issue.

Here's the improved, working mixin:

@mixin complexTransform($axis, $perspective, $degrees, $axis2, $px) {

    -webkit-transform:
        perspective( $perspective + px )
        rotate#{$axis +"(" $degrees +deg +")" }
        translate#{ $axis2 +"(" + $px +px +")" };

    -moz-transform:
        perspective( $perspective + px )
        rotate#{$axis +"(" $degrees +deg +")" }
        translate#{ $axis2 +"(" + $px +px +")" };

    -o-transform:
        perspective( $perspective + px )
        rotate#{$axis +"(" $degrees +deg +")" }
        translate#{ $axis2 +"(" + $px +px +")" };

    -ms-transform:
        perspective( $perspective + px )
        rotate#{$axis +"(" $degrees +deg +")" }
        translate#{ $axis2 +"(" + $px +px +")" };

    transform:
        perspective( $perspective + px )
        rotate#{$axis +"(" $degrees +deg +")" }
        translate#{ $axis2 +"(" + $px +px +")" };
}

Answer 1

With a little modification, I find this mixin suit my requirements better:

 @mixin keyframeBuilder($animationName, $f, $startParams, $stopParams) {
        @-webkit-keyframes $animationName {
            from { -webkit-transform: $f+'('#{$startParams}')' }
            to   { -webkit-transform: $f+'('#{$stopParams}')' }
        }
        @-moz-keyframes $animationName {
            from { -moz-transform: $f+'('#{$startParams}')' }
        to   { -moz-transform: $f+'('#{$stopParams}')' }
        }
        ... (other vendor prefixes)
    }

So essentially when I call

@include keyframeBuilder(spin, rotate, 0deg, 360deg);

It produces:

@-webkit-keyframes spin {
  from {
    -webkit-transform: rotate(0deg); }

  to {
    -webkit-transform: rotate(360deg); } }

@-moz-keyframes spin {
  from {
    -moz-transform: rotate(0deg); }

  to {
    -moz-transform: rotate(360deg); } }

@-o-keyframes spin {
  from {
    -o-transform: rotate(0deg); }

  to {
    -o-transform: rotate(360deg); } }

@keyframes spin {
  from {
    transform: rotate(0deg); }

  to {
    transform: rotate(360deg); } }

I hope this will also help someone! :)