# Parse trees

Technical Whitepaper Parse Trees and Functional Forms

A parse tree represents an expression, not immediately evaluated. Its virtue is that the expression can be evaluated whenever and in whatever context it is needed. The two main functions dealing with parse trees are eval, which evaluates a parse tree, and parse, which returns one from a string containing a valid q or k expression.

Parse trees may be the result of applying parse, or constructed explicitly. The simplest parse tree is a single constant expression. Note that, in a parse tree, a variable is represented by a symbol containing its name. To represent a symbol or a list of symbols, you will need to use enlist on that expression.

q)eval 45
45
q)x:4
q)eval x
4
q)eval enlist x
x


Any other parse tree takes a form of a list, of which the first item is a function and the remaining items are its arguments. Any of these items can be parse trees. Parse trees may be arbitrarily deep (up to thousands of layers), so any expression can be represented.

q)eval (til;4)
0 1 2 3
q)eval (/;+)
+/
q)eval ((/;+);(til;(+;2;2)))
6


## parse¶

Syntax: parse x

Where x is a string representing a well-formed q or k expression, returns a parse tree. (V3.4 can accept newlines within the string; previous versions cannot.)

The resulting parse tree can be executed with the eval function.

q)parse "1 2 3 + 5"            / the list 1 2 3 is parsed as a single item
+
1 2 3
5


This function can clarify order of execution.

q)parse "1 2 3 +/: 5 7"        / each-right has postfix syntax
(/:;+)
1 2 3
5 7
q)parse "1 2 3 +neg 5 7"       / neg is applied before +
+
1 2 3
(-:;5 7)


K expressions should be prefixed with "k)", e.g.

q)parse "k)!10"
!:
10


Use eval to evaluate the parse tree:

q)eval parse "1 2 3 +/: 5 7"
6 7 8
8 9 10


Explicit definitions in .q are shown in full:

q)foo:{x+2}
q)parse "foo each til 5"
k){x'y}
foo
(k){$[-6h=@x;!x;'type]};5)  Q-SQL statements are parsed to the corresponding functional form. q)\l sp.q q)x:parse "select part:p,qty by sup:s from sp where qty>200,p=p1" q)x ? sp ,((>;qty;200);(=;p;,p1)) (,sup)!,s partqty!pqty q)eval x sup| part qty ---| -------- s1 | p1 300 s2 | p1 300  Parsing views Views are special in that they are not parsable (sensibly) with -5!x (parse).  q)eval parse"a::5" 5 q)a 5 q)views[] symbol$()


## eval¶

Syntax: eval x

Where x is a parse tree, returns the result of evaluating it.

The eval function is the complement of parse and can be used to evaluate the parse trees it returns. (Also parse trees constructed explicitly.)

q)parse "2+3"
+
2
3
q)eval parse "2+3"
5
q)eval (+;2;3)      / constructed explicitly
5


## reval¶

Syntax: reval x

The reval function is similar to eval (-6!), and behaves as if the command-line option -b were active during evaluation.

An example usage is inside the message handler .z.pg, useful for access control, here blocking sync messages from updating:

q).z.pg:{reval(value;x)} / define in process listening on port 5000
q)h:hopen 5000 / from another process on same host
q)h"a:4"
'noupdate: . a


Partitioned databases

For partitioned databases, q caches the count for a table, and this count cannot be updated from within a reval expression or from a slave thread. Thus to avoid 'noupdate on queries on partitioned tables you should put count table` in your startup script.