Return type not correct?

[alvarotrigo]

I found this shader online and I was wondering how to implement it using gl-transitions.

uniform float mixer;
uniform sampler2D tDiffuse;
uniform sampler2D tSphere;
varying vec2 vUv;
//idea
//https://www.decarpentier.nl/lens-distortion
//distort method from 
// https://www.imaginationtech.com/blog/speeding-up-gpu-barrel-distortion-correction-in-mobile-vr/
vec2 distort(vec2 st, float alpha, float expo ){
    vec2 p1 = vec2(2.0 * st - 1.0);
    vec2 p2 = p1 / (1.0 - alpha * length( p1 ) * expo );
    return (p2 + 1.0) * 0.5;
}
void main() {
    float t = mixer;
    float tr = smoothstep( 0.75,1., t );
    float e = t * 4.;
    
    //stretch out city scene
    vec2 uv = distort( vUv, -10. * pow( .5 + t * .5, 32. ), e );
    vec4 city = texture2D( tDiffuse, uv );
    
    //pinch in sphere scene
    uv = distort( vUv, -10. * ( 1. - tr ), e );
    vec4 sphere = texture2D( tSphere, uv );
    //blend    
    gl_FragColor = mix( city, sphere, tr );
}

I've tried replacing the function content on the gl-transition editor for the noise function's content but it keeps on saying return type is not correct: