Re: [q-lang-users] where

[Eddie R. <ed...@bm...>]

Dr. Graef and (Mr? Dr?) Hubbard,

Of all of the functions in mathematics that I've ever seen, if a "where
clause" is given at the end of a piecewise function, the variables and/or
functions denoted by "where" are global to all of the "pieces." In Q's
case it is very similar to

f x = cond (Condition1, Expression1 with A;
            Condition2, Expression2 with A;
            Condition3, Expression3 with A;
            ...
            ConditionN, ExpressionN)
      where A = ...;

However, a local "where clause" for each equation is a handy feature of
Q.  In math books and papers, symbols are usually defined with "Assume
..." or "Let ..." and then go on to define a function based on the those
symbols. That is why I put a "letting" statement where I did (Although
Mr. Hubbard's use of where looks better).  Perhaps maybe:

let N = #S - 1;
strip C S = strip C $ sub S 0 (N-1) if (S!N = C);
          = ...

or

This might mess things up since "let N" looks like any other equation.
Making "let" a keyword might be a solution, but, then again, since "let
N" looks like any other equation, it doesn't stand out and might get lost
in larger definitions.

Another idea, so that wheres are nested:

f X Y Z
  where A = ... ; // global to all equations belonging to f
     = ...;
     = ...;
  where B = ... ; // global to all equations below this and belonging to f
     = ...;
     = ...;
  where C = ... where D = ...;
     = ...;



Eddie

> Rob Hubbard wrote:
>> Perhaps the "where" (and even "if" for a common conditional) keywords
>> could be used for the "letting" clauses (assuming the grammar doesn't
>> then become ambiguous, and assuming the language can be extended
>> consistently in this way).
>
> Yes, this could be done. Looks a bit weird, though:
>
> strip C S where N = #S-1
>            = strip C $ sub S 0 (N-1) if (S!N = C);
>            = strip C $ sub S 1 N     if (S!0 = C);
>            = S                       otherwise;
>
> Maybe we can invent a syntax that better mimics the way that
> mathematicians actually write these things?
>
> Also, you might then want to nest those shared definitions. E.g., have
> another shared variable which is only defined for the second and third
> equation. I don't see how this can be done in a free-format language
> without introducing some form of bulky begin ... end brackets.
>
> Of course there's an existing solution to this, namely Haskell's guarded
> equation syntax, but that also looks fairly messy IMHO. Oh well, I guess
> I'll have to take another look at Miranda to see if Turner already
> solved that problem.
>
> --
> Dr. Albert Gr"af
> Dept. of Music-Informatics, University of Mainz, Germany
> Email:  Dr....@t-..., ag...@mu...
> WWW:    http://www.musikinformatik.uni-mainz.de/ag
>
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