GLSL rejects a long list of words it does not implement, and `half` is one of them

Fact

GLSL reserves a set of identifiers "for future use" that are not keywords in the language you are actually writing. Using one is a compile error, not a warning:

ERROR: 0:20: 'half' : Illegal use of reserved word

The list is long and contains ordinary nouns — in GLSL ES 1.00 it includes half, double, long, short, input, output, interface, union, this, namespace, switch, default, goto, and the hvec/fvec/dvec vector families. Several are the obvious name for the thing being computed. half is the one that bites hardest, because the Blinn-Phong halfway vector is called the half vector in every text that describes it:

vec3 half = normalize(lightDir + viewDir);   // will not compile

Why it matters

Shader compilation failures are reported through the API rather than thrown, so whatever the program does when the shader fails is what you see. Code that falls back — to a second renderer, to a flat colour, to a 2D path — swallows the error entirely and the feature is simply absent, with the real message sitting in a getShaderInfoLog nobody read.

When the message is read, it is still misleading if the identifier is not a keyword in any language you know. Nothing about half looks reserved, so the first guess is a typo or a driver quirk rather than the spec.

How to apply

• Always read getShaderInfoLog on failure and surface it. One console.warn turns this from a mystery into a one-line fix.
• Suspect the reserved list whenever a shader rejects an identifier that is not a keyword in the language. Rename rather than investigating the driver.
• Prefer a qualified name — halfway, halfVector — over the bare noun; it is free and it survives a future spec revision that reserves more words.