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copied!<p>Defined in <code>Prelude</code>,</p> <pre><code>type ShowS = String -> String class Show a where showsPrec :: Int -> a -> ShowS show :: a -> String showList :: [a] -> ShowS type ReadS a = String -> [(a, String)] class Read a where readsPrec :: Int -> ReadS a readList :: ReadS [a] read :: (Read a) => String -> a </code></pre> <p>In short, these are the standard "serialization" methods in Haskell. <code>show :: (Show a) => a -> String</code> can turn anything that is an instance of <code>Show</code> into a string, and <code>read :: (Read a) => String -> a</code> can turn a string into anything that is an instance of <code>Read</code> (or throw an exception).</p> <p>Most of the built-in types and data structures in the standard library have <code>Show</code> and <code>Read</code> instances defined; if you're composing parts from them, your type also has <code>Show</code> and <code>Read</code> instances defined.</p> <pre><code>type Table = [(String, String)] load :: (Read a) => FilePath -> IO a load f = do s <- readFile f return (read s) save :: (Show a) => a -> FilePath -> IO () save x f = writeFile f (show x) </code></pre> <p>If <code>Table</code> were a datatype, you have to ask for the instances, but you can request that the compiler automatically derive them for you.</p> <pre><code>data Table = Table [(String, String)] deriving (Read, Show) </code></pre> <p>Sometimes that's not possible and you have to define your own instances.</p> <pre><code>instance Show Table where showsPrec p x = ... instance Read Table where readsPrec p x = ... </code></pre> <p>But that shouldn't be common.</p>

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