Shader of SkMesh

[João Oliveira]

Whilst using SkMesh to render 3D geometries I attempted to use backface culling, hence I need to discard the pixel if the normal of the vertex is not in view of the camera. 

These are my shaders: 

    const char* vs = R"(

        uniform float4x4 proj_model;

        uniform float4x4 model;

        uniform float3 camera_forward;

        Varyings main(const Attributes a) {

            Varyings v;

            float4 clip = proj_model * float4(a.position, 1.0);

            float3 ndc = clip.xyz / clip.w;

            v.position = (ndc).xy;

            v.lighting = clamp(dot(a.normal, camera_forward), 0.0, 1.0);

            v.v_color = a.color;

            return v;

        }

    )";

    const char* fs = R"(

        float2 main(const Varyings v, out half4 color) {

            color = half4(v.v_color.rgb * v.lighting, v.v_color.a);

            return v.position;

        }

    )";

    auto result = SkMeshSpecification::Make(

        SkSpan(attributes),

        sizeof(Vertex),

        SkSpan(varyings),

        SkString(vs),

        SkString(fs)

    );

 Yet when I use the keyword discard in the fragment shader, the SKSL compiler complains that I can only use it when compiling a fragment shader? A bit of clarity if I am attempting do to something illegal with the SkMesh would be helpful