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copied!<p>You're writing a parsing function. The basic type would be something like <code>String -> Tree Int</code>, but there are a few powerful refinements worth considering.</p> <p>First, not all <code>String</code>s are actually translatable into <code>Tree</code>s (i.e. <code>")"</code> is definitely not a tree) so it's better to refine the function to something with a type like <code>String -> Maybe (Tree Int)</code>, the <code>Maybe</code> monad indicates possible failure.</p> <p>Secondly, you probably don't really want to reinvent an <code>Int</code> parser. You can use the <code>Read</code> typeclass to get access to a collection of parsers. So you might</p> <ul> <li>refine the type again then as <code>Read a => String -> Maybe (Tree a)</code></li> <li>import the <code>Safe</code> module to get access to <code>readMay :: Read a => String -> Maybe a</code></li> <li>and then just build the <code>Tree</code> parsing components.</li> </ul> <p>Thirdly, <code>Tree</code>s are highly recursive, your parenthetical Lispy grammar is highly recursive, your parser ought also to be. To this end, focus on just parsing a single node <code>"(...)"</code> and then recursively parsing the inner nodes as needed.</p> <p>Finally, you might want to eventually consider some even more powerful parsing tools. <code>Parsec</code> is much more powerful than the <code>Read</code> class and makes building more complex, recursive parsers from smaller, simpler pieces a snap.</p>

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