I am going to tell you something fantastic, but first, I want to explain some things about this:
us:{$[#i:&{(y~*K)&"*"~*x}':x;@[x;i;:[;,"_"]];x]}
The first is that there's a typo in what bidirectional wrote. The above is correct. The second, is what it is. Once I have explained that, I can tell you the fantastic thing.
k syntax is very simple. There's just a few forms you need to be aware of:
f x
which applies x to f.
a f b
which is apply f to the two arguments a and b, and:
f[a;b;c]
which allows you to do three arguments. You can write the first one as f[x] and the second as f[a;b] if you like even more consistency. f can be an "operator" -- that is a symbol. The symbols ' / and \ are special and called adverbs. These adverbs have a special form if followed by a colon, so ': is different than ' and has nothing to do with : or '. I think Arthur just ran out of keys on the keyboard. Once you have those, parenthesis () and braces {} have some special syntax, just like double-quotes " do.
With the syntax explained, let us try to understand what we are looking at.
us: is how we start assignment. You can say "us gets" if you like (the colon can be pronounced). {} braces surround a lambda, this one takes a single argument "x" (the first argument). $[a;b;c] is cond like in lisp; if a then b else c. # means count. i: is another assignment.
& means where -- the argument to which is going to be a bitmap like 000100b or 01101b or something like that, and where returns the indices of the set bits; the former example being the list 3, the latter example being the three-element list 1 2 4.
Another lambda comes next: We can see it takes two arguments because there's an x and a y in there (y is the second argument). We can get a clue as to what it expects because the following adverb ': means each-prior. This tells us "x" is going to be a list of things, and this lambda is going to consume them pairwise. If given the list {(x;y)}':"iliketacos" we get the result:
{(x;y)}':"iliketacos"
i
li
il
ki
ek
te
at
ca
oc
so
y is the "previous" value, and "x" is the current value. The "where" before it tells us we want to know the indices where the condition inside is true. Let's try and understand that condition.
y~*K is in parenthesis. Parenthesis group, so we execute them first (just like in other languages). We're looking for a situation where the previous value is the first (that's what asterisk means here) of K. What is K?
K:("select";"distinct";"partition";"from";"where";"group";"having";"order";"limit")
So we're looking for a value (x) whose previous (y) is the first of K which is "select". The "&" that follows here is "and" - Arthur likes to overload operators since there aren't many symbols on the keyboard and this is something you get used to.
So you can read {(y~*K)&"*"~*x}':x as simply trying to find the sequences "select star" -- given a list ("select"; "*"; "from"; "potato") you get 0100b and from ("select"; "*"; "from"; "("; "select"; "*"; "from"; "potato; ")") you get 01000100b. I think the attempt is to disambiguate the asterisks in the sql:
select * from tacos where cat=4*42
but sql is a strange and irregular language, so this kind of thing is necessary. Back to our query:
us:{$[#i:&{(y~*K)&"*"~*x}':x;@[x;i;:[;,"_"]];x]}
i is going to be the locations of the asterisks following select. If the count of that is nonzero; we're going to do the @-part, and if not, we're just going to return x.
@[x;i;f]
is called amend. It returns x, but at indices i, we apply them to f, so it's x[i]:f[x[i]] which is pretty cool. f in this case is a projection, of "gets" (the function colon) with the second-argument bound to an underscore. That is:
:[;,"_"]
is just a function. That's how @[x;i;:[;,"_"] replaces all of the asterisks that follow select with an a "_"
Almost. It's actually a list of length one, rather than the scalar "_". I haven't read everything in sql.k but this is probably important elsewhere.
Ok. Now that I have explained what this is and what it does, I am ready to tell you something fantastic. I read this:
us:{$[#i:&{(y~*K)&"*"~*x}':x;@[x;i;:[;,"_"]];x]}
as:
"us gets a function, that finds the indices of asterisk following the first element of K, and then replaces the things at those indices with underscores"
Literally. From left, to right. Just that fast. And I only program in k part-time. That's not the fantastic thing. The fantastic thing is that by learning to read k, I am almost miraculously able to read other languages faster. This:
copied=False
for i in range(1,len(a)):
if a[i] == "*" and a[i-1] == "select":
if not copied:
copied = True
a = a[:]
a[i] = "_";
gives me some grief for being so irregular and gross, and I have to look up range/xrange and len and memorise a much more complex set of rules for syntax, and I have to track the order of things carefully and so on, but I have places in my brain, made by k, for those things, and so I am able to absorb code in other languages faster.
If that does not amaze you, I do not think you have considered the ramifications of what I said. I can suggest maybe reading it again (or maybe actually reading what I wrote instead of skipping to the punchline), but if after two or three tries you are still lost, maybe you can ask a question and I can try to answer it.