Using exceptions in imperative code

29 Jul 2003

      I've got resource acquisition (and release) code which looks like this at
present:
...
connect = do
  envHandle <- envCreate
  errHandle <- handleAlloc oci_HTYPE_ERROR envHandle
  serverHandle <- handleAlloc oci_HTYPE_SERVER envHandle
  connection <- dbLogon "user" "password" "database" envHandle errHandle
  session <- getSession errHandle connection
  return (envHandle, errHandle, serverHandle, connection, session)
...
disconnect envHandle errHandle serverHandle connection session = do
  sessionEnd errHandle connection session
  serverDetach errHandle serverHandle
  handleFree oci_HTYPE_SERVER serverHandle
  handleFree oci_HTYPE_ERROR errHandle
  handleFree oci_HTYPE_ENV envHandle
  terminate
How should I structure it to handle exceptions raised by the various
functions (envCreate, handleAlloc, dbLogon, etc)? I've started to
restructure it like this:

getEnv = do
  catchDyn ( do
    envHandle <- envCreate
    getErr envHandle
  ) (putStrLn "envCreate failed")

getErr envHandle = do
  catchDyn ( do
    errHandle <- handleAlloc oci_HTYPE_ERROR envHandle
    getServer envHandle errHandle
  ) (do
    reportOCIException envHandle
    handleFree oci_HTYPE_ENV envHandle
  )

getServer envHandle errHandle = do
  catchDyn ( do
    serverHandle <- handleAlloc oci_HTYPE_SERVER envHandle
    getServer envHandle
  ) (do
    reportOCIException envHandle
    handleFree oci_HTYPE_ERROR errHandle
    handleFree oci_HTYPE_ENV envHandle
  )
...

but this is so awkward. How should I structure it?

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Using exceptions in imperative code

Bayley, Alistair