destructuring: as patterns?

Claus Reinke claus.reinke at talk21.com
Wed Apr 18 10:16:45 PDT 2012


> I've found it quite useful in Clojure/Script and I'm sure folks who have
> encountered the feature in the ML derived languages would agree.

Indeed, all of the pattern-match-supporting functional languages
I've used also supported 'as'-patterns in some form ('var at pat' in 
Haskell,  'var as pat' in SML and variants, 'pat=pat' in Erlang, 
'as pat var' in (little-known) KiR).

Those constructs are hard to search for on github (too many
false matches, though searching for 'as' in Standard ML code
might work), but I find them both useful and popular. 

Some of the usefulness comes from using pattern matching 
as a structural guard (checking the substructure conforms to 
a pattern, then just naming the substructure wholesale). I 
have no experience with a language that only destructures, 
without matching, but I would expect to use as-patterns 
with destructuring as well, increasingly so with upcoming
language constructs.

The main problems seem to be cover-grammars (construct
should be valid expression and pattern), and that the obvious
choice '=' is already taken for default parameters. 

A common generalization of as-patterns are conjunctive 
patterns (as in Successor ML or Erlang), which suggests 
'pat&&pat' or perhaps '(pat,pat)' as alternative syntax.

Claus
 


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