[Haskell-cafe] Re: How to simplify this code?

Very nice series of refactorings! I'd like to add that it might be a better argument order to replace: JSON a => MyData -> String -> a -> MyData with: JSON a => String -> a -> MyData -> MyData Just so you can get a (MyData -> MyData) transformer, which is often useful. Eyal On Jan 16, 1:52 am, "Ryan Ingram" wrote:

Here's a series of refactorings that I feel gets to the essence of the code.

For reference, here's the original.

...

add :: JSON a => MyData -> String -> a -> MyData add m k v = fromJust $ (return $ json m) >>= jsObj >>= (return . fromJSObject) >>= (return . ((k, showJSON v):)) >>= (return . toJSObject) >>= (return . showJSON) >>= \js -> (return $ m { json = js })

-- turn into do notation add :: JSON a => MyData -> String -> a -> MyData add m k v = fromJust $ do         t1 <- return $ json m         t2 <- jsObj t1         t3 <- return $ fromJSObject t2         t4 <- return ( (k, showJSON v) : t3 )         t5 <- return $ toJSObject t4         js <- return $ showJSON t5         t6 <- return $ m { json = js }         return t6

-- replace "var <- return exp" with "let var = exp" add :: JSON a => MyData -> String -> a -> MyData add m k v = fromJust $ do         let t1 = json m         t2 <- jsObj t1         let t3 = fromJSObject t2         let t4 = (k, showJSON v) : t3         let t5 = toJSObject t4         let js = showJSON t5         let t6 = m { json = js }         return t6

-- inline some small definitions add m k v = fromJust $ do         t2 <- jsObj (json m)         let js = showJSON $ toJSObject ((k, showJSON v) : fromJSObject t2)         let t6 = m { json = js }         return t6

-- there's only one real "Maybe" object in here, and we fromJust afterwards, -- so put the "can't fail" assumption in the right place. -- -- This is the only refactoring that I felt was at all "tricky" to figure out. add m k v =         let t2 = fromJust $ jsObj (json m)             js = showJSON $ toJSObject ((k, showJSON v) : fromJSObject t2)             t6 = m { json = js }         in t6

-- sugar let, inline t6 add m k v = m { json = js } where     t2 = fromJust $ jsObj (json m)     js = showJSON $ toJSObject ((k, showJSON v) : fromJSObject t2)

-- inline t2 add m k v = m { json = js } where     js = showJSON $ toJSObject ((k, showJSON v) : fromJSObject (fromJust $ jsObj (json m)))

-- uninline dictionary entry add m k v = m { json = js } where     js = showJSON $ toJSObject (newEntry : fromJSObject (fromJust $ jsObj (json m)))     newEntry = (k, showJSON v)

-- factor out modification modifyJSON f m = m { json = f (json m) } add m k v = modifyJson go m where     go js = showJSON $ toJSObject (newEntry : fromJSObject (fromJust $ jsObj js))     newEntry = (k, showJSON v)

-- turn into pipeline modifyJSON f m = m { json = f (json m) } add m k v = modifyJSON go m where     go js = showJSON $ toJSObject $ (newEntry :) $ fromJSObject $ fromJust $ jsObj js     newEntry = (k, showJSON v)

-- pointless modifyJSON f m = m { json = f (json m) } add m k v = modifyJSON go m where     go = showJSON . toJSObject . (newEntry :) . fromJSObject . fromJust . jsObj     newEntry = (k, showJSON v)

Final result:

...

modifyJSON f m = m { json = f (json m) }

...

add m k v = modifyJSON go m where     go = showJSON . toJSObject . (newEntry :) . fromJSObject . fromJust . jsObj     newEntry = (k, showJSON v)

Some stylistic choices are debatable (pointless vs. not, inline vs. not), but I think this is a lot more readable than the >>= and liftM madness you had going.

I also might refactor the (fromJSObject --> some transformation --> toJSObject) path; this seems like a fundamental operation on "MyData", but I don't know enough about the library you are using to suggest the direction to go with this.

  -- ryan

On Thu, Jan 15, 2009 at 11:14 AM, Levi Greenspan

wrote:

...

Dear list members,

...

I started looking into monadic programming in Haskell and I have some difficulties to come up with code that is concise, easy to read and easy on the eyes. In particular I would like to have a function "add" with following type signature: JSON a => MyData -> String -> a -> MyData. MyData holds a JSValue and add should add a key and a value to this JSON object. here is what I came up with and I am far from satisfied. Maybe someone can help me to simplify this...

...

module Test where

...

import Text.JSON import Data.Maybe (isJust, fromJust) import Control.Monad

...

data MyData = MyData { json :: JSValue } deriving (Read, Show)

...

jsObj :: JSValue -> Maybe (JSObject JSValue) jsObj (JSObject o) = Just o jsObj _ = Nothing

...

add :: JSON a => MyData -> String -> a -> MyData add m k v = fromJust $ (return $ json m) >>= jsObj >>= (return . fromJSObject) >>= (return . ((k, showJSON v):)) >>= (return . toJSObject) >>= (return . showJSON) >>= \js -> (return $ m { json = js })

...

add2 :: JSON a => MyData -> String -> a -> MyData add2 m k v = fromJust $ (\js -> m { json = js }) `liftM` (showJSON `liftM` (toJSObject `liftM` (((k, showJSON v):) `liftM` (fromJSObject `liftM` (jsObj $ json m)))))

...

add3 :: JSON a => MyData -> String -> a -> MyData add3 = undefined -- How to simplify add?

...

What the code essentially does is that using functions from Text.JSON, it gets the list of key-value pairs and conses another pair to it before wrapping it again in the JSValue-Type.

...

Many thanks, Levi _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe