Fragment shader compiling in webGL but not in OpenGL

Question

I am programming in Haxe (language compiling to multiple platforms) and I have written some shaders.

My fragment shader runs fine in html5, but when I try to compile for native (OS X and/or Neko, a VM for Haxe) I get a shader compilation error, but no details (I am using lime which is a platform abstraction that does these things for me).

Here is the shader:

precision mediump float;

varying vec4 v_color;

void main() {
    gl_FragColor = v_color;
}

Very simple as you can see. It runs fine in webGL, but it seems it won't compile in OpenGL. I am no expert in shaders so I have no idea what might be wrong. Am I using some syntax that only exists in webGL?

Also just in case, here is my vertex shader (which compiles fine):

attribute vec3 a_position;
attribute vec4 a_color;
uniform mat4 uMVMatrix;
uniform mat4 uPMatrix;

varying vec4 v_color;

void main() {
    gl_Position = uMVMatrix * vec4(a_position, 1);
    v_color = a_color;
}

Answer 1

As suggested in the comments, the precision directives are not supported in the OpenGL version that haxe/lime compiles to.

Adding a #version 130 doesn't work either, maybe haxe or lime doesn't read these I don't know.

Anyway I found the solution reading through the lime samples:

#if !desktop
"precision mediump float;" +
#end
"varying vec4 v_color;

void main() {
    gl_FragColor = v_color;
}

That's not really beautiful but it works. If someone knows a way to get precision in desktop shaders, that would be better though.

Answer 2

WebGL is based on OpenGL ES. Precision qualifiers (like "precision mediump float;") were introduced in OpenGL ES to optimize performance on embedded systems with lower hardware (like smartphones). In desktop OpenGL these precision qualifier do nothing. They only exist for compatibillity to OpenGL ES / WebGL. Source: https://www.opengl.org/wiki/Type_Qualifier_%28GLSL%29#Precision_qualifiers

Pehraps you need a newer version of desktop gl to support those qualifiers?