[Haskell-beginners] Implementing Read

Sean Perry shaleh at speakeasy.net
Mon Sep 12 00:21:00 CEST 2011

I see this has been answered, but since someone else may be learning too I thought I would share.

I have always used deriving for Read so I was unsure. This post gave me the push to go hunt it down. A very quick grep for "instance Read" in the ghc lib sources and I found:

instance Read Int64 where
    readsPrec p s = [(fromInteger x, r) | (x, r) <- readsPrec p s]

In the Int.hs source. The other ints are similar. So like Daniel I ended up with a definition based on reads once I experimented quickly.

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