Explanation of macros; Haskell macros

[Pascal Costanza]

Dirk Thierbach wrote:

> Since the overhead of evaluating it at runtime is minimal, especially
> with lazyness, that's exactly the situation where it is natural to use
> a HOF instead of a macro.

OK, nice solution.

> I didn't go through the two proposed Lisp solutions in detail, but here's
> a HOF in Haskell that does the same. First a general 'cond' with an
> explicit default case, then 'econd' based on cond with an error as
> default case:
> 
> cond :: a -> [(Bool, a)] -> a
> cond def []             = def
> cond def ((True,x):_)   = x
> cond def ((False,_):xs) = econd xs
                             ^^^^^^^^

I guess this is a typo - it should be "cond xs", right?

> econd :: [(Bool, a)] -> a
> econd = cond (error "Default econd case")

Hmm, what if I had wanted to add a default check in front of each cond, 
instead of at the end?

Also, Coby's version prints a useful error message that mentions all 
conditions ("could not satisfy..."):

> CL-USER 91 > (econd
>                 ((= 3 4) "foo")
>                 ((= 4 4) "bar"))
> "bar"
> 
> CL-USER 92 > (econd
>                 ((= 3 4) "foo")
>                 ((= 4 5) "bar"))
> 
> Error: fell through ECOND form.  could not satisfy any of the following:
> (= 3 4)
> (= 4 5)

Would it be possible to add such a message with your proposed technique?


Pascal