Dear Pipers (Pipe Smokers?),
Please accept my apologies for a lengthy example for what is undoubtedly
a simple problem, but I'm struggling to know how best to simplify.
I'm writing a little command-execution package. It's primarily for my
own learning, so don't worry about "couldn't I use package X...".
I have a 'systemx' function, that creates a command that will be run
with stdout/stderr inherited from the process:
systemx :: (MonadError ExecError μ, MonadReader (ProcExecCtxt3 μ) ρ)
=> CmdSpec -> ρ (Producer [CmdSpec] μ ())
A CmdSpec is a command specification - that is, an executable name &
some arguments.
ProcExecCtxt3 holds an 'executor' - a function to 'execute' the command;
in normal circumstances, this uses System.Process et al, but it's
replaceable so that I could
mock the command. Hence the free typevar μ, which is MonadIO in normal
circumstances but could be a non-IO monad for mock purposes. The
ExecError is thrown if the command returns non-zero.
The producer is to "log" the commands run - when mocking, I can use a
MonadWriter to collect the list, and use that for testing.
Now, this all works fine in simple form, where runCtxt3 is mechanics for
executing via System.Process:
cmds1 :: (MonadError ExecError μ, MonadReader (ProcExecCtxt3 μ) ρ) => ρ
(Producer [CmdSpec] μ ())
cmds1 = systemx (CmdSpec grep ["root", "/etc/passwd"])
cmds1' :: (MonadError ExecError μ, MonadReader (ProcExecCtxt3 μ) ρ) => ρ
(Producer [CmdSpec] μ ())
cmds1' = systemx (CmdSpec grep ["foo", "/etc/motd"])
λ> :t runCtxt3
runCtxt3 :: MonadIO μ => ProcExecCtxt3 μ
λ> runExceptT $ runEffect $ for (runReader (cmds1') runCtxt3) (mapM_
warnCmd)
CMD: /bin/grep foo /etc/motd
/bin/grep: /etc/motd: No such file or directory
Left (ExecError (ExitVal 2))
λ> runExceptT $ runEffect $ for (runReader cmds1 runCtxt3) (mapM_ warnCmd)
CMD: /bin/grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
Right ()
BUT! If I attempt to execute two commands, only the last one gets run:
cmds2 :: (MonadError ExecError μ, MonadReader (ProcExecCtxt3 μ) ρ) => ρ
(Producer [CmdSpec] μ ())
cmds2 = do
systemx grepIt
systemx grepIt2
This is baffling me. I would suspect laziness, but that we're clearly
running within MonadIO, that doesn't apply, right?
I'm sure I'm being a bit dim, but any pointers would be gratefully received.
Yours,
Martyn.
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