Instructions
Report network IO statistics.
You are writing a PaaS, and you need a way to bill customers based on network and filesystem usage.
Create a wrapper for network connections and files that can report IO statistics.
The wrapper must report:
- The total number of bytes read/written.
- The total number of read/write operations.
Keeping count in Roc
Your wrapper handles the bookkeeping; the supplied backend does the actual reading and writing.
There are no files or connections to open yourself.
Paasio(state, err) works with any backend state and error types.
The backend holds its initial state and two callbacks:
-
read!(state, max_bytes) returns { state, result }, with Ok(bytes) or Err(error).
-
write!(state, bytes) returns { state, result }, with Ok(bytes_written) or Err(error).
Start each wrapper's four counters at zero.
create, stats, and get_state must not invoke the callbacks.
For each read! or write!, call the matching callback exactly once, passing its arguments unchanged.
Keep the returned state and result, even on failure, and return { io, result }.
Carry that updated io into the next call.
Every attempt counts
Keep read and write statistics separate:
- Count every call, including failures, EOF, zero-byte reads, and empty writes.
- Count bytes only on success: the returned list's length for reads, and the reported number for writes.
- A short transfer still counts as one call—don't retry it or a failed operation.
Count what the backend reports; you don't need to track operating-system calls or buffering.
You can trust the backend's byte counts and assume the counters fit in U64.
Running the tests
The tests use both in-memory backends and a real file in a temporary directory, which is cleaned up afterward.
Unlike most other exercises, this exercise uses effectful functions (in this case based on the basic-cli platform).
If you want to run the tests locally, you must therefore use roc --opt=speed paasio-test.roc, rather than roc test (since the expect statement cannot currently test effectful functions).
Any error returned by the Roc code is reported by the platform, using a different format than usual.
Hints
Show hints
Try storing the backend and counters together in the opaque type.
To call a function stored in a record field, use parentheses: (backend.read!)(backend.state, max_bytes).
Watch out for ?: returning early on an error would lose the updated wrapper and its operation count.
