I have been hard at work on the
shell-monad
ever since it was born on Christmas Eve. It's now up to 820 lines of code,
and has nearly comprehensive coverage of all shell features.
Time to use it for something interesting! Let's make a shell script and a
haskell program that both speak a simple protocol. This kind of thing could
be used by
propellor when it's deploying itself to a new host. The
haskell program can ssh to a remote host and run the shell program, and
talk back and forth over stdio with it, using the protocol they both speak.
abstract beginnings
First, we'll write a data type for the commands in the protocol.
data Proto
= Foo String
Bar
Baz Integer
deriving (Show)
Now, let's go type class crazy!
class Monad t => OutputsProto t where
output :: Proto -> t ()
instance OutputsProto IO where
output = putStrLn . fromProto
So far, nothing interesting; this makes the IO monad an instance of the
OutputsProto type class, and gives a simple implementation to output a
line of the protocol.
instance OutputsProto Script where
output = cmd "echo" . fromProto
Now it gets interesting. The Script monad is now also a member of the
OutputsProto. To output a line of the protocol, it just uses echo.
Yeah -- shell code is a member of a haskell type class. Awesome -- most
abstract shell code evar!
Similarly, we can add another type class for monads that can input the
protocol:
class Monad t => InputsProto t p where
input :: t p
instance InputsProto IO Proto where
input = toProto <$> readLn
instance InputsProto Script Var where
input = do
v <- newVar ()
readVar v
return v
While the IO version reads and deserializes a line back to a Proto,
the shell script version of this returns a Var, which has the newly
read line in it, not yet deserialized. Why the difference? Well, Haskell
has data types, and shell does not ...
speaking the protocol
Now we have enough groundwork to write haskell code in the IO monad that
speaks the protocol in arbitrary ways. For example:
protoExchangeIO :: Proto -> IO Proto
protoExchangeIO p = do
output p
input
fooIO :: IO ()
fooIO = do
resp <- protoExchangeIO (Foo "starting up")
-- etc
But that's trivial and uninteresting. Anyone who has read to here certianly
knows how to write haskell code in the IO monad. The interesting part is
making the shell program speak the protocol, including doing various things
when it receives the commands.
foo :: Script ()
foo = do
stopOnFailure True
handler <- func (NamedLike "handler") $
handleProto =<< input
output (Foo "starting up")
handler
output Bar
handler
handleFoo :: Var -> Script ()
handleFoo v = toStderr $ cmd "echo" "yay, I got a Foo" v
handleBar :: Script ()
handleBar = toStderr $ cmd "echo" "yay, I got a Bar"
handleBaz :: Var -> Script ()
handleBaz num = forCmd (cmd "seq" (Val (1 :: Int)) num) $
toStderr . cmd "echo" "yay, I got a Baz"
serialization
I've left out a few serialization functions.
fromProto is used in both
instances of OutputsProto. The haskell program and the script will both use
this to serialize Proto.
fromProto :: Proto -> String
fromProto (Foo s) = pFOO ++ " " ++ s
fromProto Bar = pBAR ++ " "
fromProto (Baz i) = pBAZ ++ " " ++ show i
pFOO, pBAR, pBAZ :: String
(pFOO, pBAR, pBAZ) = ("FOO", "BAR", "BAZ")
And here's the haskell function to convert the other direction, which was also
used earlier.
toProto :: String -> Proto
toProto s = case break (== ' ') s of
(w, ' ':rest)
w == pFOO -> Foo rest
w == pBAR && null rest -> Bar
w == pBAZ -> Baz (read rest)
otherwise -> error $ "unknown protocol command: " ++ w
(_, _) -> error "protocol splitting error"
We also need a version of that written in the Script monad. Here it is.
Compare and contrast the function below with the one above. They're
really quite similar. (Sadly, not so similar to allow refactoring out a common
function..)
handleProto :: Var -> Script ()
handleProto v = do
w <- getProtoCommand v
let rest = getProtoRest v
caseOf w
[ (quote (T.pack pFOO), handleFoo =<< rest)
, (quote (T.pack pBAR), handleBar)
, (quote (T.pack pBAZ), handleBaz =<< rest)
, (glob "*", do
toStderr $ cmd "echo" "unknown protocol command" w
cmd "false"
)
]
Both
toProto and
handleProto split the incoming line apart into the
first word, and the rest of the line, then match the first word against the
commands in the protocol, and dispatches to appropriate actions. So,
how do we split a variable apart like that in the Shell monad? Like this...
getProtoCommand :: Var -> Script Var
getProtoCommand v = trimVar LongestMatch FromEnd v (glob " *")
getProtoRest :: Var -> Script Var
getProtoRest v = trimVar ShortestMatch FromBeginning v (glob "[! ]*[ ]")
(This could probably be improved by using a DSL to generate the globs
too..)
conclusion
And finally, here's a
main to generate the shell script!
main :: IO ()
main = T.writeFile "protocol.sh" $ script foo
The pretty-printed shell script that produces is not very interesting,
but I'll include it at the end for completeness. More interestingly for the
purposes of sshing to a host and running the command there, we can use
linearScript to generate a version of the script that's all contained on
a single line. Also included below.
I could easily have written the pretty-printed version of the shell script
in twice the time that it took to write the haskell program that generates
it and also speaks the protocol itself.
I would certianly have had to test the hand-written shell script repeatedly.
Code like
for _x in $(seq 1 "$ _v#[!\ ]*[\ ] ") doesn't just write and
debug itself. (Until now!)
But, the generated scrpt worked 100% on the first try! Well, it worked as
soon as I got the Haskell program to compile...
But the best part is that the Haskell program and the shell script
don't just speak the same protocol. They both rely on the same
definition of Proto. So this is fairly close to the kind of type-safe
protocol serialization that Fay provides, when compiling Haskell to
javascript.
I'm getting the feeling that I won't be writing too many nontrivial shell
scripts by hand anymore! :)
the complete haskell program
Is here,
all 99 lines of it.
the pretty-printed shell program
#!/bin/sh
set -x
_handler () :
_v=
read _v
case "$ _v%%\ * " in FOO) :
echo 'yay, I got a Foo' "$ _v#[!\ ]*[\ ] " >&2
: ;; BAR) :
echo 'yay, I got a Bar' >&2
: ;; BAZ) :
for _x in $(seq 1 "$ _v#[!\ ]*[\ ] ")
do :
echo 'yay, I got a Baz' "$_x" >&2
done
: ;; *) :
echo 'unknown protocol command' "$ _v%%\ * " >&2
false
: ;; esac
echo 'FOO starting up'
_handler
echo 'BAR '
_handler
the one-liner shell program
set -p; _handler () :; _v=; read _v; case "$ _v%%\ * " in FOO) :; echo 'yay, I got a Foo' "$ _v#[!\ ]*[\ ] " >&2; : ;; BAR) :; echo 'yay, I got a Bar' >&2; : ;; BAZ) :; for _x in $(seq 1 "$ _v#[!\ ]*[\ ] "); do :; echo 'yay, I got a Baz' "$_x" >&2; done; : ;; *) :; echo 'unknown protocol command' "$ _v%%\ * " >&2; false; : ;; esac; ; echo 'FOO starting up'; _handler; echo 'BAR '; _handler
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