Re: "dict"ionary typing?

On Wed, Dec 4, 2013 at 9:09 PM, Richard A. O'Keefe <[email protected]>wrote:

> Lee Naish had a functional preprocessor.
>
> I had one called 'funnel', which relied on declarations.
> If a name/arity pair was declared as a function or if
> there was a :- func [mod:]name/arity declaration, then
> in a clause body
>         p(...,name(T1,...,Tn),...)
> =>
>         name(T1, ..., Tn, T0),
>         p(..., T0, ...)
>
> Of course, if anyone wants to add functional syntax
> in Prolog, they _really_ need to look at Mercury,
> which does not introduce special syntax for function
> calls but relies on declarations.
>

I'll also describe my experience writing and working with functional syntax
in Prolog.  Jan may find it useful as he mulls over functional notation for
V7.

I really enjoy how library(func) allows one to use most predicates, without
declaration, in a functional context.  In this case, the $ operator
(because math) does the heavy lifting.  I find it powerful to be able to
compose these predicates to form other, sometimes anonymous, predicates
using the 'of' operator.  I use function application in nearly all my
Prolog code these days; mostly as a way to avoid variable name
proliferation.  I don't use function composition nearly as often as I
thought I would.  That's partly because Prolog doesn't yet have a rich
library of functional building blocks like Haskell has.

I also like the way that library(func) can use a variety of terms as if
they were functions.  For example, one obtains Python-style string
interpolation by treating strings as functions.

I dislike declarations when they can be inferred from usage.  I
particularly dislike function declarations in Prolog because they imply
that a predicate can only be used as a function in a single "mode".  One
might reasonably use append/3 as a function in three different modes.  I
haven't implemented it yet, but I'm considering a new feature for
library(func) inspired by Ciao's notation.  Specifically, a single ~ marks
the argument to be used as output for *this* call.

  ?- writeln(append([a,b],[c,d],~)).
  [a,b,c,d].

  ?- length(List, ~ is 1 + 1).
  List = [_G1821, _G1824].

-- 
Michael
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