Provides a more flexible getArgs function with better error reporting.

Latest on Hackage:0.2.1

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow to host generated Haddocks.

LGPL licensed by Ketil Malde
Maintained by Ketil Malde

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┬╣

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
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

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. Here's a custom
data type (which is parsed using its Read instance, and also needs

import System.SimpleArgs
import Data.Typeable

data Gender = Male | Female deriving (Typeable,Read)

main = do
g <- getArgs
case g of Male -> putStrLn "X and Y"
Female -> putStrLn "two Xs"

Instead of reporting errors rather anonymously as 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

Depends on:
Used by 3 packages:
comments powered byDisqus