A reactive Hydra parameter must be written as a () => … thunk — a bare expression is evaluated once at eval time and frozen

In Hydra, parameter values that should change over time (including a.fft reads) must be wrapped in a () => arrow function called a thunk. Hydra evaluates thunks every frame. A bare expression — even one referencing a.fft[0] — is evaluated exactly once when the sketch is eval()-ed and its result is stored as a constant. The pattern () => 0.6 + a.fft[0] * 2.5 is reactive; 0.6 + a.fft[0] * 2.5 is frozen at the value of a.fft[0] at boot time. This is the single most common ‘why isn’t it reacting?’ bug.

Examples

osc(10,0.1,()=>0.6+a.fft[0]*2.5) — brightness updates every frame ✅. osc(10,0.1, 0.6+a.fft[0]*2.5) — brightness is the value of a.fft[0] at the moment of eval, then frozen ❌.

Assessment

A student writes osc(20,0.1,1+a.fft[0]).out(). The sketch compiles without error but the oscillator brightness never changes. Identify the bug, explain why it occurs, and write the corrected line.