[Haskell-cafe] Advice for clean code.

Jules Bean jules at jellybean.co.uk
Tue Dec 4 08:05:44 EST 2007

Felipe Lessa wrote:
> On Dec 4, 2007 1:28 AM, Don Stewart <dons at galois.com> 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.

I don't think it's hugely important though.


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