explain this custom function, please?

[halifaxious]

I found a function to replace text using a lookup table. It works great. But the syntax is unfamiliar to me and I can't find a reference for it.  Here's what I have:

replacer = (x,y,z)=>LookupTable{[#"lookup value"=x]}?[replacement]? ??x

I have 3 questions about the above:

1. why does the =x go inside the column reference? LookupTable{[#"lookup value"=x]}
2. where is the documentation for what looks like a ternary operator in M?  a?b? ??c
3. why doesn't the column reference in the true portion of the statement need to be fully justified with the table name? [replacement]

[m_dekorte · Accepted solution]

Hi @halifaxious,

There is a lot going on here

replacer = (x,y,z)=>LookupTable{[#"lookup value"=x]}?[replacement]? ??x

First this is a custom replacer function which takes 3 arguments: 

x = original value

y = match condition (true or false)

z = replacement value

Next its applying (optional) key match lookup to identify a unique row in the table

LookupTable{ [#"lookup value"=x] }?

Followed by (optional) field access, to return a value from a field/ column

[replacement]?

And finally applying coalesce, to return the input value if a null is returned.

?? x

Ps. If this helps solve your query please mark this post as Solution, thanks!

Some links to resources on these topics: 

Visual guide to key match lookup 

Visual guide to coalesce 

Item- and Field access 

[halifaxious]

Thank you for a very illuminating answer! I wish I'd come here about 2 hours ago instead of fruitlessly googling.