[Haskell-cafe] Re: Debugging partial functions by the rules

[Jón Fairbairn]

Robert Dockins <robdockins at fastmail.fm> writes:

> On Nov 15, 2006, at 9:48 AM, Jón Fairbairn wrote:
> > But instead of “blah (head [ v | (n,v) <- env, n==target ])
> > blah”, you could write
> >
> > blah the_v_in_scope blah
> > where (the_v_in_scope:_) =  [ v | (n,v) <- env, n==target ]
> 
> Or how about.... ??
> 
> lookupVarible target env =
>     case [ v | (n,v) <- env, n==target ] of
>        (x:_) -> x
>        _ -> assert False $ "BUG: Unexpected variable out of
> scope "++ (show target)++" in environment "++(show env)
> 
> 
>   ... lookupVariable target env ....
> 
> 
> It seems to me that every possible use of a partial function
> has some  (possibly imagined) program invariant that
> prevents it from failing.   Otherwise it is downright wrong.
> 'head', 'fromJust' and friends  don't do anything to put
> that invariant in the program text.

Hear hear.

-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk