kdb:解析"x [y]"的类型是什么? [英] kdb: what is the type of parse"x[y]"?
问题描述
对于表达式 parse"x [y]" ，我认为这表示对参数 y x 的函数/映射/列表应用>，可能是投影.如果我错了，请纠正我.
For expression parse"x[y]", I think it means a function/map/list application of x on argument y, possibly a projection. Please correct me if I'm wrong.
现在进入Q控制台，我得到以下输出.
Now entering into Q console, I get below output.
q)parse"x[y]"
x
y
q)l:parse"x[y]"
q)count l
2
q)type l
0h
q)l[0]
`x
q)l[1]
`y
q)type l[0]
-11h
q)type l[1]
-11h
我们看到 l 的类型为 0h .它的长度为2.两个元素的类型均为 -11h .为什么是列表，而不是 11h 类型?
We see l has type 0h. It has length 2. Both elements have type -11h. Why is the list then, not of type 11h?
推荐答案
简而言之，它是一个解析树签名'type'构造，与lambda(类型为64)非常相似.当对象到达调用堆栈时， k 会在解析树中识别并评估表达式，从而取消引用符号的作用，就像指针一样.
In short, it's a parse tree signature 'type' construct, very similar to lambda (which would be type 64). k recognises and evaluates expression in parse tree when object reaches call stack, de-referencing symbols-which acts like a pointer.
TL; DR -逐步细分
- 基本评估结构
"x [y]"〜"x y"
q)x:count
q)y: 1 2 3
q)parse "x y"
x
y
q)x y
3
q)`x y
3
q)eval parse "x y"
3
简单索引，简化" 第二个参数->内部原子:
Simple indexing, 'simplified' 2nd param -> int atom:
q)parse "y 2" //this nicely shows parse tree
`y
2
q)eval parse "y 2"
3
q)eval (`y;2)
3
q)`y 2
3
q)y 2
3
以上两个示例都是混合列表(0h)，实际元素紧随其后-其中符号是引用.而是使用普通的混合列表构造，以后将定义所有长度和类型
Both above examples are mixed lists (0h), with actual elements following immediately - where symbols - are references. Instead with normal mixed list construct where all lenghts and types are defined later
- 着眼于序列化的IPC框架有助于理解这种结构
确定哪些表达式相同
q)parse["x[y]"] ~ parse "x y"
1b
q)parse["x[y]"] ~ parse "x@y"
0b
q)parse["x[y]"] ~ parse "{x y}"
0b
比较字节帧
q)-8!parse "x[y]"
0x0100000014000000000002000000f57800f57900
q)-8!parse "x y"
0x0100000014000000000002000000f57800f57900
q)-8!`x`y
0x01000000120000000b000200000078007900
q)-8!enlist `x`y
0x01000000180000000000010000000b000200000078007900
将第一段的解析树输出与实际obj而不是引用进行比较
q)-8!parse "x y"
0x0100000014000000000002000000f57800f57900
q)-8!parse "x 2"
0x010000001a000000000002000000f57800f90200000000000000
Lambda结构以完成列表以供参考
q)-8!{x y}
0x010000001500000064000a00050000007b7820797d
/0x7b "{" ; 0x7d "}"
- 最后是IPC框架故障
`x`y(符号列表)
q)symlist:`endian`isync`x`y`length`type`attr`msg!sums[0 1 1 1 1 4 1 1] _ -8!`x`y
q)symlist
endian| ,0x01 (little endian)
isync | ,0x00
x | ,0x00
y | ,0x00
length| 0x12000000
type | ,0x0b (11h - symbol list - 1st difference)
attr | ,0x00
msg | 0x0200000078007900
深入研究对称列表:
q)msg:`lenght`msg!sums[0 4] _ symlist`msg
q)msg
lenght| 0x02000000
msg | 0x78007900
在这里我们可以清楚地看到，由于显式类型:0x0b-11h-符号列表，空终止符号如何立即跟随向量的长度
Here we can clearly see, how null terminated symbol follows length of vector immediately, due to explicit type: 0x0b - 11h - symbol list
q)`char$msg`msg
"x\000y\000"
解析"x [y]"；-解析树故障
q)o1:`endian`isync`x`y`length`type`attr`msg!sums[0 1 1 1 1 4 1 1]_ -8!parse"x[y]"
q)o1
endian| ,0x01
isync | ,0x00
x | ,0x00
y | ,0x00
length| 0x14000000
type | ,0x00 (could expect 64 - 100h lambda)
attr | ,0x00
msg | 0x02000000f57800f57900
与简单列表的区别:
q)`lenght`type1`msg1`type2`msg2!sums[0 4 1 2 1 ] _ o1`msg
lenght| 0x02000000
type1 | ,0xf5 (-11h - symbol atom) - acts as a reference
msg1 | 0x7800 null terminated "x"
type2 | ,0xf5 (-11h - symbol atom)
msg2 | 0x7900 null terminated "y" - acts as a reference
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