[Haskell-cafe] Re: Making monadic code more concise

[Alexander Solla]

On Nov 16, 2010, at 12:36 PM, Ling Yang wrote:

>  Are only
> 'trivial' results possible, or that the incomputability problems are  
> just moved
> into type space?

That's typically the case, under Rice's theorem.

A construct is derivable if it works for all cases (i.e., it's a "free  
theorem"), or if it works for none.  If it works for some, you need to  
encode the differences between the cases yourself.