[Haskell-cafe] read with pattern

Cale Gibbard cgibbard at gmail.com
Thu Apr 21 20:09:23 EDT 2005

It's not really clear to me what you're asking. The function read is a
method of the typeclass Read, and it has type Read a => String -> a.

There are a number of instances of the typeclass Read in the standard
prelude (http://www.haskell.org/onlinereport/standard-prelude.html)

instance  Read Int  where ...
instance  (Read a, Read b) => Read (a,b)  where ...
instance  (Read a) => Read [a]  where ...
which, taken together, imply that the type [(Int,Int)] is in the
typeclass Read, and so there is a suitable implementation of read ::
String -> [(Int,Int)]  If you want to see how that might be
implemented (though haskell implementations are allowed to do the same
thing in a different way), see the link to the prelude there.

hope this helps,
 - Cale

On 4/21/05, Walt Potter <potter at southwestern.edu> wrote:
> I'd like to find a reference to reading with patterns such as
> read::[(Int,Int)]
> Thanks, Walt

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