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    <p>The error that is causing your compile problems is that you are directly casting an expression to the type <code>Either (IO ()) (String, [String])</code> when it is not an <code>Either</code> value. (The compiler is not outputting a very helpful error message.)</p> <p>To create an Either value [1], we use the data constructors <code>Left</code> and <code>Right</code>. Convention (from the library page) is that errors are a Left value, while correct values are a Right value.</p> <p>I did a quick rewrite of your arg checking function as </p> <pre><code>checkArgs :: [String] -&gt; Either String (String, [String]) checkArgs args = case args of [] -&gt; Left "No arguments given. See --help for more info." _ -&gt; let (params@(param:_),rest) = span (\(c:_) -&gt; c=='-') args in if length params &gt; 1 then Left ("No arguments given for " ++ param) else let (pArgs,_) = span (\(c:_) -&gt; c/='-') rest in Right (param, pArgs) </code></pre> <p>Note that the arg checking function does not interact with any external <code>IO ()</code> library functions and so has a purely functional type. In general if your code does not have monadic elements (<code>IO ()</code>), it can be clearer to write it in purely functional style. (When starting out in Haskell this is definitely something I would recommend rather than trying to get your head around monads/monad transformers/etc immediately.)</p> <p>When you are a little more comfortable with monads, you may want to check out <code>Control.Monad.Error</code> [2], which can wraps similar functionality as <code>Either</code> as a monad and would encapsulate some details like <code>Left</code> always being computation errors.</p> <p>[1] <a href="http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Data-Either.html" rel="nofollow">http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Data-Either.html</a></p> <p>[2] <a href="http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Monad-Error.html" rel="nofollow">http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Monad-Error.html</a></p>
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    1. COhm.. thanks. apparently when I define checkArgs in a purely functional form instead of inside main using 'Either (IO ()) (String, String) works fine :P -- http://paste.pocoo.org/show/344101/
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