Re: [Haskell-cafe] Advice for clean code.

4 Dec 2007


      jules:
...
Felipe Lessa wrote:
...
On Dec 4, 2007 1:28 AM, Don Stewart  wrote:
...
-- How to display results
   instance Show Action where
       show MoveOutOfBounds    = "Sorry you can't move in that 
       direction."
       show (MoveBadTerrain a) = case a of
                                 Wall      -> "You walk into a wall."
                                 Tree      -> "There is a tree in the 
                                 way."
                                 otherwise -> "You can't move there."
       show MoveOk             = "Good move."
I always thought show was meant for returning a String that could be
used to recreate the original data if you copy-pasted it in your code
or if you used read (i.e. read . show == id). Reading the
documentation more carefully, I see that [1] says that this property
holds for *derived* instances, and says nothing about it in the
general case.
I would not write what dons wrote.
I would have a custom function here rather than "misusing" Show in this 
way; call it "showMoveError" or similar. For mostly the reasons Felipe gave.
Yes, this is considered bad practice in larger project (not so much in
little hacks), since read . show should hold, as should the 'paste'
property.

-- Don