𓆤DBay SQL Macros

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• 𓆤DBay SQL Macros
• To Do

𓆤DBay SQL Macros

Because User-Defined Functions have several shortcomings in SQLite, below), an alternative mechanism named dbay-sql-macros has been conceived to work around some of those issues.

dbay-sql-macros has been integrated into DBay such that when one constructs the db instance as db = new DBay { macros: true, }, all statements will be subjected to macro expansion implicitly before SQLite gets to see them. That said, dbay-sql-macros can be used independently from DBay as all it is concerned with is really interpolation of values into a given string, according to the definition of macros it has recorded.

Instances of dbay-sql-macros hav two public methods, declare() and resolve() to define and use macros. An example:

{ DBay_sqlx }     = require 'dbay-sql-macros'
m                 = new DBay_sqlx()
m.declare "@secret_power( @a, @b ) = power( @a, @b ) / @b;"
result  = m.resolve "select @secret_power( 3, 2 ) as p;"

In the above, we have declared a function-like macro named @secret_power() that accepts two arguments, @a and @b. To the right of the equals sign we see power( @a, @b ) / @b; which is, except for the trailing semicolon, the 'body' of the macro, which is what the macro, when used in an SQL query, will 'resolve' or 'expand' to, with values interpolated to replace the parameters in the body. Schematically and step by step:

select @secret_power( 3, 2 ) as p; -- original query w/ macro
--     |-------------------|       -- only this stretch of the query is affected
select power( @a, @b ) / @b  as p; -- the query with the macro body inserted
select power(  3, @b ) / @b  as p; -- the query with the macro body inserted
select power(  3,  2 ) / 2   as p; -- the query with the macro body inserted
--     |-------------------|

Notes:

• The use of the @ (at-sign) in the above is purely a convention to avoid name clashes with existing SQLite keywords and identifiers (@ is not allowed by SQLite in identifiers, so should be safe).
• The exact syntax for macro declarations is still under consideration and may change.
• In particular, one wants to allow multiple statements to appear in macros.
• As it stands, everything to the right hand side of the equals sign minus any trailing semicolon becomes part of the body.
• The underlying SQLite DB never gets to see the declarations, only the resolved SQL.
• Therefore, the DB file remains valid for sufficiently compatible software like the SQLite Command Line Tool.
• Some effort has been put into parsing parameters and arguments and parentheses pairs. As a happy result, it is possible to put unpaired parentheses into string literals without confusing the mechanism, and to nest function and macro calls, so this is legal and will resolve correctly: select @secret_power( foo( x ), @bar( y, z, ')' ) ). However the dbay-sql-lexer is still in its incipient stage so hiccups can not be ruled out.
• Parameters and arguments must always match in length. Only macros with constant arities are currently supported.
• It is possible to declare and use macros without the parentheses.
• A macro that has been declared with empty parentheses may be called with empty or without parentheses, and vice versa.
• Macros are resolved recursively in way that allows to use macros in macro bodies and macro calls.

m.declare SQL"""@add( @a, @b ) = ( @a + @b );"""
m.declare SQL"""@mul( @a, @b ) = ( @a * @b );"""
#.........................................................................................................
do ->
  probe   = SQL"""select @add( @mul( @add( 1, 2 ), 3 ), @add( 4, @mul( 5, 6 ) ) ) as p;"""
  matcher = 'select ( ( ( 1 + 2 ) * 3 ) + ( 4 + ( 5 * 6 ) ) ) as p;'

To Do

• [–] documentation
• [–] safeguard against using a macro in its own expansion, leads to infinite regress