How to do build variants in shake? - shake-build-system

Question

How can pass parameters in shake and then use them in our rules? I would like to make some equivalents of:
make ARCH=x86_64
or
make DEBUG=YES
etc...
For now i've been using enviroment variables and getEnv function to simulate these.
How can i make a dependency on a given parameter (environment variable) so that it can be read once in a build, store it somewhere and access it from multiple rules?
e.g. For now i am doing getEnv multiple times in multiple rules but during compilation time the enviroment variable can change. Also for example a debug flag can alter both compiler and linker flags meaning that the info would be needed to be available in both link and compile output rules.

Answer 1

Your existing approach of using environment variables should work, and since environment variables are already tracked by Shake, one approach is to parse DEBUG=YES and turn it into an environment variable. For example:
main = shakeArgsWith shakeOptions [] $ \_ args -> do
let (vars,files) = partition ('=' `elem`) args
forM_ vars $ \v -> let (a,'=':b) = break (== '=') v in setEnv a b
return $ Just $ if null files then rules else want files >> withoutActions rules
rules :: Rules ()
rules = ...
Since environment variables are local to a process (and their child processes), this will probably work fine.
Of course, storing such information in the environment variables is a bit ugly. You can create your own oracle to store the information instead:
main = shakeArgsWith shakeOptions [] $ \_ args -> do
let (vars,files) = partition ('=' `elem`) args
let vars2 = [(a,b) | v <- vars, let (a,'=':b) = break (== '=') v]
return $ Just $ do
global <- addOracle $ \(Global x) -> return $ lookup x vars2
if null files then rules global else want files >> withoutActions (rules global)
newtype Global = Global String deriving (Show,Typeable,Eq,Hashable,Binary,NFData)
rules :: (Global -> Action (Maybe String)) -> Rules ()
rules global = undefined
Now, instead of writing the information into an environment variable with setEnv, we store it in an oracle with addOracle, which will still be tracked and not conflict with any other environment-variable pieces.