cannot assign to a derived cell (`let<signal>`)

Why this fires¶

TE720 reports an assignment to a derived signal declared with let<signal>. A derived cell is defined by its initializer and is recomputed from the source cells it reads; it is not writable state.

var<signal> price = 30
var<signal> quantity = 2
let<signal> total = price * quantity

total = 10
// error[TE720]: cannot assign to a derived cell

Allowing the assignment would make total disagree with its dependency graph until the next recomputation.

Fix it¶

Update one of the writable source cells:

price = 5
quantity = 2
// total is now derived as 10

If the value must be independently writable, declare it as var<signal> and own the synchronization logic explicitly. If it is purely computed, keep let<signal> and change only its inputs.

See also¶

• TE719 — an island handler cannot be lowered to JavaScript.
• TE721 — an island effect cannot be lowered to JavaScript.