Question

Why does this:

Manipulate[test[a_] := 2*b; test[c], {b, 0, 1}, {c, 0, 1}]

turns into an evaluation loop?
Shouldn’t Manipulate only evaluate when b or c changes?

Answer 1

Yes, the Manipulate will re-evaluate when b or c changes, but also if test changes — and test is being re-assigned every time any one of those values changes. Hence the endless re-evaluation loop.

As a rule, side-effects should be avoided in the display expressions of constructs like Manipulate and Dynamic in order to avoid evaluation loops, race conditions and other surprising behaviour. In the case at hand, I would suggest removing the implicit dependency on b in test and hoisting its definition outside of the Manipulate:

test[b_, c_] := 2*b; Manipulate[test[b, c], {b, 0, 1}, {c, 0, 1}]

In the real application, there may be obstacles to such a simple refactoring — but the key point is to remove the := from the dynamic expression.