New ErlSQL Feature: Lisp-style Operator Expansion

When you first write a library, you really don't know how useful it can be. You often discover new ways of enhancing it and making it more powerful only when you start using it. This has been the case with almost every library I wrote: Smerl, ErlyDB, and now ErlSQL.

While I was using ErlSQL to hack ErlyDB's internals, I realized what a pain it is to use the SQL way of writing repeated expressions that use the same binary operator, e.g. {{{a,'=',b}, 'and', {c,'=',d}}, 'and', {e,'=',f}}. Lisp handles such situations much more elegantly than SQL (I know, it's hard to believe that anything could be more elegant than SQL, but what do you know :) ). In Lisp, you would write (and (= a b) (= c d) (= e f)), which spares you having to write the 'AND' operator for each new element in the list.

(Please forgive me if I made any egregious Lisp syntax errors in this example. I haven't touched Lisp since college and my knowledge of it is very rusty :) )

The Lisp way is often much more concise than the SQL way, so I implemented a Lisp-style operator expansion feature in ErlSQL.

Here are a few examples of how to use this feature:


{select,'*',{from,foo},
{where,{a,'=',{'+',[1,2,3]}}}} ->
"SELECT * FROM foo WHERE (a = 1 + 2 + 3)"

{select,'*',{from,foo}
{where,{'=',[{'+',[a,b,c]},{'+',[d,e,f]}]}}} ->
"SELECT * FROM foo WHERE a + b + c = d + e + f"

{select,'*',{from,foo},
{where,
{'and',[{a,'=',b},{c,'=',d},{e,'=',f}]}}} ->
"SELECT * FROM foo WHERE (a = b) AND (c = d) AND (e = f)"


Any expression which is a tuple where the first item is an operator and the second item is a list will be expanded by ErlSQL in a similar fashion.