> One of the best bad example is the use of boolean as arguments.
Oh, yes. That's a pet peeve of mine. About 99% of boolean arguments
should be meaningful two-valued enumerated types. It's literally a
one-liner to create such an enumerated type, so there's no excuse.
The documentation effect and type safety provided by two-valued
enumerated types is indeed much greater. But one needs a conversion from
Bool to the enumeration if one wants to pass the result of a logic
operation to the function. What about records with named fields,
especially if more options are needed?
data CreateDirectoryOptions = Cons {createParents :: Bool}
createDirectory (Cons {createParents = True}) "dir"
Hummmm.... I think I like this. Something like the following allows a
simple way to make the call site concise and provide defaults at the
same time. Additional plus -- adding options requires no call-site code
changes.
---------------------------------------
import Control.Monad
-- provide "opts" which should be a labeled record
-- of default options
class FunctionOptions a where opts :: a
-- alias for readability when you don't want to change
-- the default options
defaults = opts
-- create a datatype for each function which needs some flags
data CreateDirectoryOptions
= CreateDirectoryOptions
{ createParents :: Bool
, barFlag :: Bool
, andNow :: Bool
, aWholeBunch :: Bool
, ofOther :: Bool
, seldomEverUsed :: Bool
, esotericOptions :: Bool
}
-- set the flag defaults
instance FunctionOptions CreateDirectoryOptions
where opts =
CreateDirectoryOptions
{ createParents = False
, barFlag = True
, andNow = True
, aWholeBunch = True
, ofOther = False
, seldomEverUsed = True
, esotericOptions = False
}
createDirectory :: CreateDirectoryOptions -> FilePath -> IO ()
createDirectory o path =
do when (createParents o) (putStrLn "creating parents")
when (barFlag o) (putStrLn "bar flag true")
putStrLn ("creating "++path)
return ()
-- readable AND typesafe :-)
main = do createDirectory opts{ createParents = True } "foo"
createDirectory defaults "baz"
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