问题
In the answers for the tutorials for OCaml available at this site, some of the solutions, including the one for eliminating consecutive duplicates of list elements, is written as such:
let rec compress = function
| a :: (b :: _ as t) -> if a = b then compress t else a :: compress t
| smaller -> smaller;;
What is the relevance of the line a :: (b:: _ as t)? Why can't I write it as a :: b :: t instead?
回答1:
The t in b :: _ as t is bound to b :: _. So the meaning is different. If you use the pattern a :: b :: t you would need to say compress (b :: t), which is a little less elegant and a tiny bit less efficient.
回答2:
The keyword as binds a name to all or part of a pattern. Once bound, the name can be used instead of the pattern it represents. In your "compress" function, t is bound to the pattern b :: _.
Once t is bound, it can be used in subsequent expressions, as in the rest of the "compress" function.
as name binding occurs left-to-right, unlike most languages (excepting C's typedef). Also, :: appears to have higher precedence than as.
Therefore, (b :: _ as t) is equivalent to ((b :: _) as t). This can be confusing for those used to right-to-left bindings. Note that a :: (b :: _) as t will bind the whole pattern a :: b :: _ to t, due to the precedence mentioned above.
Reference:
- http://caml.inria.fr/pub/docs/manual-ocaml/patterns.html#hevea_manual.kwd7
- http://caml.inria.fr/pub/docs/oreilly-book/html/book-ora016.html.
来源：https://stackoverflow.com/questions/26769403/as-keyword-in-ocaml