When indexing dictionary in depth I've found different results in the same (as I think) constructions:
q)d:`a`b!(1 2 3;4 5 6)
q)d[`a`b;0]
1 4
q)d[`a`b]0
1 2 3
Why is this happening? How q understands and distinguishes two different cases? Before this I was confident that, for example, calling dyadic function f[a;b] and f[a]b are the same. And now I am not sure even about this.
To index at depth you either need semi colons separating your arguments, or use the dot. Your second example,
d[`a`b] 0
Is taking the 2 lists from the dictionary values and then indexing to return the first. While
d[`a`b;0]
or
d .(`a`b;0)
Is taking the 2 lists, and then indexing at depth, taking the first element of each, due to the semi colon/dot
When you call a dyadic function it is expecting two parameters, passing one inside the square brackets creates a projection, which is basically using an implicit semi colon, so
f[a]b
is the same as
f[a;]b
which is the same as
f[a;b]
The result of
f[a]
is a projection which is expecting another argument, so
f[a] b
evaluates f[a], then passes argument b to this function, with usual function application via juxtaposition
Your dictionary indexing example does not create a projection, and hence the indexing is not expecting any more arguments, so the first indexing
d[`a`b]
is evaluated immediately to give a result, and then the second index is applied to this result. It would work the same for a monadic function
q){5+til x}[5] 2
7
Like the top level dictionary index, the application is carried out and then the result is indexed, as only one argument was expected, with no projection involved