52Implementation

Pass a polymorphic vector width

Problem

A polymorphic BDPI import would receive its vector through a pointer and get no automatic width, so this design keeps the C function concrete: foreignWidth takes one 8-bit width. The polymorphic BSV helper vectorWidth must supply that width for each Bit#(n) type used by the adapter. The starter returns zero for every type, so the public run prints width = 0 for a 17-bit value.

Change only the body of vectorWidth. Keep its signature, the interface with its three methods, the adapter, the foreign-function declaration, and mkTop.

Read and predict

1. Read vectorWidth's signature. The width lives in the type parameter n, not in the argument's bits.
2. Read mkWidthAdapter. The same definition is used at three types: Bit#(5), Bit#(17), and Bit#(31). Each use fixes n separately.
3. Before pressing Run, predict the result for each of the three widths, and explain why the argument's bit pattern cannot answer the question: 17'h0 and 17'h1ffff have the same width.

Make the repair

Turn the type-level number into a value, then convert it to the 8-bit result. Replace only the returned expression, run the 17-bit public path, and then reason through the other two instantiations without running them.

Completion contract

• vectorWidth returns n as an 8-bit value for the adapter's widths 5, 17, and 31.
• All values of one type produce the same width, whatever their bits.
• The three adapter methods report 5, 17, and 31.
• Returning 17 as a constant matches only the public sample.

Progressive hints

Transfer experiment

Call one adapter method with all zeros and then with all ones. Predict why both report the same width before running, then restore the original call.