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

On Tue, 4 Dec 2007, Dougal Stanton wrote:

On 04/12/2007, Felipe Lessa wrote:

...

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.

So, what's the deal here? May I use Show for anything without breaking conventions? And how about Read?

That seems to be convention though I don't think it's required for the correct operation of anything. Anyone?

You will find that convention comfortable if you work in GHCi - may it be just for debugging. In turn I think it is even good to leak results of 'show' to the outside world, but due to lack of alternatives I'm doing it myself regularly. Since 'show' is intended to show internals of a data structure (and this is what 'deriving Show' implements) this will leak internal information to the user. But the user does not know the internals of your program, does not know your function and constructor names, so they are not of much help for him. And if the function names tell something to the user, he might still want to get them presented in his mother's tongue. I encountered e.g. the following problem: I printed numbers for processing by a different program. GHC's 'show' returned '1.0e-2', Hugs' 'show' returned '0.01' - both are correct Haskell literals, but the postprocessing program didn't understand '1.0e-2'. That is 'show' does not give you much control on the output format that you need for reliable post-processing.