Double comma when converting qsql statements to functional statements

[jason_fealy]

The list of constraints (index 2) is itself a parse tree which means value cannot be applied to the output of parse. For a simple example like this, it’s just a matter of removing a level of nesting but just to illustrate….

q)show t:([]a:1 2;b:10 20); 
a b 
---- 
1 10 2 20 

q)show pt:parse"select from t where a>1"; 
// parse tree at index 2 
? `t ,,(>;`a;1) 0b () 

q)value pt // value cannot be applied 
'type [0] value pt // value cannot be applied ^ 

q)@[pt;2;eval] // evalute the parse tree at index 2 which removes a level of nesting 
? `t ,(>;`a;1) 0b () 

q) 

q)value @[pt;2;eval] // value can be applied to output 
a b 
---- 
2 20 

q) 

q)?[t;enlist(>;`a;1);0b;()] ~ value @[pt;2;eval] 
// matches manually typed functional form 1b

 

Some further reading which includes more complex examples https://code.kx.com/q/wp/parse-trees/