Building ? using kleene closure {not haskell specific}
Hello gentle Haskell folks, I happened to read "Beautiful code"'s chapter 1 today and found Brian Kerninghan's regex implementation. In it he only shows the * meta character. I can easily understand how + can be built but am having trouble with building ? (zero or one). I'd really appreciate it if some one could help me understand it. Regards, Kashyap
Hi,
Perhaps I did not understand the question properly, but it looks very
straight forward :
oneOrNone x = fmap Just x <|> pure Nothing
it will have a type of oneOrNone :: (Alternative f) => f a -> f (Maybe a)
In fact, there is a function called "optional" in Control.Applicative[1]
which does exactly that.
Is this what you are looking for?
thanks,
Hemanth K
[1]
http://haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Control-Appli...
On Fri, Aug 12, 2011 at 1:31 PM, C K Kashyap
Hello gentle Haskell folks,
I happened to read "Beautiful code"'s chapter 1 today and found Brian Kerninghan's regex implementation. In it he only shows the * meta character. I can easily understand how + can be built but am having trouble with building ? (zero or one). I'd really appreciate it if some one could help me understand it.
Regards, Kashyap
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I can easily understand how + can be built but am having trouble with building ? (zero or one).
If there is a regular expression e for the empty word, one can define ? as a? = e | a If there is a regular expression o that never matches one can define e as e = o* If there are character classes one can define o as o = [] Apart from that, I have no idea.. Sebastian
On Fri, Aug 12, 2011 at 4:34 PM, Sebastian Fischer
I can easily understand how + can be built but am having trouble with building ? (zero or one).
If there is a regular expression e for the empty word, one can define ? as
a? = e | a
If there is a regular expression o that never matches one can define e as
e = o*
If there are character classes one can define o as
o = []
Apart from that, I have no idea..
Sebastian
Thanks Sebastian ... this is what I was asking for. I'll try and digest it. Regards, Kashyap
participants (3)
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C K Kashyap -
Sai Hemanth K -
Sebastian Fischer