> To make an example, again ignoring infix stuff, given "a b c d e", I know to read the whole expression as application of function a to arguments b, c, d, e.

> To make an example, again ignoring infix stuff, given "a b c d e", I know to read the whole expression as application of function a to arguments b, c, d, e. Yes, but what does it really tell you? It doesn't tell you that the final result is a primitive value, it might as well be a function again. (We need to be careful how we define an "return argument", whether it is only primitive types, or also function types, and based on the chosen definition, the ambiguity is present in both types of syntax, or in neither.) Similarly, if I have a program "a b c d e" in concatenative language, it is a function from stack to stack, and we don't know, how many arguments it either took from or left at the stack when it's done, without looking at the types. The point is, ambiguity regarding number of processed arguments is there in both cases. Except in concatenative languages, it also applies to the returned arguments (i.e. a "function" in a concatenative language can kinda take negative number of arguments). I think what mainly confuses you is that you really don't need an AST with concatenative languages. And it cannot be created, it's not a tree, more like "abstract syntax sequence".