These are interesting ideas. I really like the idea of using @ as an analog to Ruby's splat. After thinking about it for a few minutes I think I like (string:@map ...) too, though I think it's starting to look like line noise.
If you compose a function with a value what does that mean? Something like ((fn (y) (prn y) y) x)? In the example you gave I would still expect to wrap prn:x is parens to execute the resulting procedure.
you are correct about the parens on prn:x... I thought I had a clever trick from making them unecessary, but it didn't pan out when I sat down to work out the necessary transformations... creating decent language primitives is hard...
...your "line noise" comment also is a good point...
This stuff is hard. I looked into implementing general splicing functionality but it is not trivial. Reading @bar and translating it to (splice 'bar) is easy enough, but then you would have to examine the arguments of every sexp before evaluating it to handle the splice. I guess it's not particularly difficult but it would likely be a performance hit. Perhaps there is a clever trick to do general splicing.
prn:x should mean [prn (x _)] because x is no different from prn, and may very well be a function. Unless you can statically know the value of x, but statical analysis of variable content does not sound very lispy to me. While it's ok for optimization it is not ok for semantics!
