Talk:Haskell/Advanced monads

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Contents 1 Signature of lookup 2 List monad example 2.1 Possible answer 3 Problem with list monad example do-notation implementation of find3rdConfig?

[edit] Signature of lookup

The signature of function lookup is given as:

lookup :: a        -- a key
       -> [(a, b)] -- the lookup table to use
       -> Maybe b  -- the result of the lookup

I don't want to be picky here, but perhaps it could be pointed out that 'a' actually has to be an equality type. Furthermore, a simple implementation of lookup could serve a better understanding of that particular example. --Xian 15:43, 18 March 2007 (UTC)

[edit] List monad example

I think there is something wrong with this example:

getNextConfigs :: Board -> [Board]
getNextConfigs = ... -- details not important

tick :: [Board] -> [Board]
tick bds = concatMap getNextConfigs bd

find3rdConfig :: Board -> [Board]
find3rdConfig bd = tick $ tick $ tick bd

First, tick has an argument "bds" but uses "bd". Second, find3rdConfig calls tick with a Board, but tick needs an argument of type [Board]. Or am I missing something here? --Stw 21:21, 13 August 2006 (UTC)

[edit] Possible answer

I would guess that that the example should be corrected as follows:

tick :: [Board] -> [Board]
tick bds = concatMap getNextConfigs bds

find3rdConfig :: Board -> [Board]
find3rdConfig bd = tick $ tick $ tick [bd]

Thanks for pointing this out -- Kowey 07:20, 15 August 2006 (UTC)

[edit] Problem with list monad example do-notation implementation of find3rdConfig?

Something doesn't feel right with the do-notation implementation of find3rdConfig. It seems like after each call to getNextConfigs, some kind of flattening (e.g. using concatMap) is necessary.

That is exactly what the list monad does:

 instance Monad [] where
   return a = [a]
   xs >>= f = concat (map f xs)

-- Felix C. Stegerman 21:46, 15 June 2007 (UTC)