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

15 Nov 2006


      Robert Dockins  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@cl.cam.ac.uk