问题
In SML, is it possible for you to have multiple patterns in one case statement?
For example, I have 4 arithmetic operators express in string, "+", "-", "*", "/"
and I want to print "PLUS MINUS"
of it is "+" or "-"
and "MULT DIV"
if it is "*" or "/"
.
TL;DR: Is there somewhere I can simplify the following to use less cases?
case str of
"+" => print("PLUS MINUS")
| "-" => print("PLUS MINUS")
| "*" => print("MULT DIV")
| "/" => print("MULT DIV")
回答1:
Given that you've tagged your question with the smlnj tag, then yes, SML/NJ supports this kind of patterns. They call it or-patterns and it looks like this:
case str of
("+" | "-") => print "PLUS MINUS"
| ("*" | "/") => print "MULT DIV"
Notice the parentheses.
The master branch of MLton supports it too, as part of their Successor ML effort, but you'll have to compile MLton yourself.
val str = "+"
val _ =
case str of
"+" | "-" => print "PLUS MINUS"
| "*" | "/" => print "MULT DIV"
Note that MLton does not require parantheses. Now compile it using this command (unlike SML/NJ, you have to enable this feature explicitly in MLton):
mlton -default-ann 'allowOrPats true' or-patterns.sml
回答2:
In Standard ML, no. In other dialects of ML, such as OCaml, yes. You may in some cases consider splitting pattern matching up into separate cases/functions, or skip pattern matching in favor of a shorter catch-all expression, e.g.
if str = "+" orelse str = "-" then "PLUS MINUS" else
if str = "*" orelse str = "/" then "MULT DIV" else ...
回答3:
Expanding upon Ionuț's example, you can even use datatypes with other types in them, but their types (and identifier assignments) must match:
datatype mytype = COST as int | QUANTITY as int | PERSON as string | PET as string;
case item of
(COST n|QUANTITY n) => print Int.toString n
|(PERSON name|PET name) => print name
If the types or names don't match, it will get rejected:
case item of
(COST n|PERSON n) => (* fails because COST is int and PERSON is string *)
(COST n|QUANTITY q) => (* fails because the identifiers are different *)
And these patterns work in function definitions as well:
fun myfun (COST n|QUANTITY n) = print Int.toString n
|myfun (PERSON name|PET name) = print name
;
来源:https://stackoverflow.com/questions/36347681/multiple-patterns-in-1-case