Readme for simpleargs-0.1
SimpleArgs - provide a more flexible and informative replacement for getArgs
For "real" command line programs, you usually want to provide a
flexible command line with various options and settings, sensibly
named and with auto-generated help. In that case, SimpleArgs is not
for you, stop reading this, and look up System.Console.GetOpt
instead.
But sometimes, a quick hack is just what you need. Previously, you
were wont to do:
main = do
[count',gender'] <- getArgs
let count = read count
let gender = case gender' of
"M" -> 'M'
"F" -> 'F'
main_real count gender
This is somewhat tedious, wastes precious sceen estate, users
supplying parameters of the wrong type will get obscure errors, and
while any programming errors you might introduce probably will be
trivial, it would be better to avoid them entirely.
The SimpleArgs module provides getArgs with an overloaded return type,
so that command line parameters are parsed as the types required by
the rest of the program.
Using SimpleArgs, the above could therefore look like this:
main = do
(count,gender) <- getArgs
main_real count gender
or even (I think):
main = getArgs >>= return . uncurry main_real
If that was a bit contrieved, let's say you just want to read a file
name:
main = do
[filename] <- getArgs
readFile filename >>= print . length
I'm sure you could avoid the information-free name "filename" by some
esoteric tranformation to more point-free style, but I argue that
SimpleArgs makes this natural and easy:
main = getArgs >>= readFile >>= print . length
I don't think 'wc -c' gets much easier than this.
Instead of reporting incomplete cases or read failures, SimpleArgs
will provide more sensible error reporting. (To try this, build Example
by executing 'ghc --make Example.hs'). It will:
1) report incorrect number of parameters,
also mentioning the expected parameters and types:
% ./Example foo
Example: Incorrect number of arguments, got 1,
expected 2 (Int,[Char])
This also gives you a useful hint if you just run the program
without any parameters.
2) report parameters that fail to parse as the required type:
% ./Example foo 10
Example: Couldn't parse parameter "foo" as type Int
Nice, huh? Please enjoy, and let me know how you fare at ketil@malde.org.