34Implementation

Build a bounded rule expression

Problem

countToBound should construct a reusable Rules value for a supplied four-bit register and bound. Its empty rules block is a valid value, so the starter can compile, but addRules installs no behavior. The public trace therefore leaves count at zero instead of ramping to four and holding there.

Add only the missing rule inside the existing expression. Preserve the function parameters, Rules return type, addRules call, observation rules, and supplied mkTop.

Read, predict, and implement minimally

Read the file in two time domains. During elaboration, countToBound(count, 4) constructs a Rules value and addRules installs it. During simulation, the installed rule evaluates its guard and may schedule a register write each cycle.

Before editing, predict the public trace: advance can print successive cycle values, but the empty collection cannot change count. Then identify the only editable structural gap—the space between rules and endrules.

Add one named rule whose guard distinguishes “below the bound” from “at the bound.” Its body should schedule one increment. Avoid adding a second register, putting the boundary test inside an always-enabled body, or moving behavior into mkTop; those changes defeat the reusable rule-expression contract.

After Run, inspect the full sequence rather than only its last line. Starting from zero with bound four, entry states 0, 1, 2, and 3 should schedule the next four states. At entry state 4, the installed rule must be disabled, so later observations remain exactly four.

Observable completion contract

• For a bound of four, the count advances one per firing from zero to four and then remains four across multiple later cycles.
• A smaller and a larger bound must both work through the same function; no literal bound belongs in the returned rule.
• A zero bound permits no increment, because the initial state is already at the boundary.
• The counter never overshoots its supplied bound.

Progressive hints

Transfer experiment

After the required version works, use a scratch copy to install the same function twice for two registers with different bounds. Predict both ramps. This demonstrates that one rule-building function can elaborate distinct rule instances. Restore the supplied mkTop before submitting.