Hacker News Reader

Comment by dfalzone | Hacker News Reader

This is pretty minor but I like how in Rust you can use constructs like if-statements and match-statements as expressions. For example:

  let foo = if some_condition {
      "condition is true"
  } else {
      "condition is false"
  };

  let bar = match x {
      0 => "string",
      1 => "another string",
      _ => "default"
  };

It's not super significant but I think it's an elegant syntactic feature.

View on HN

You could do this in GNU C probably at least as far back as 1990. In GNU C, this syntax:

  ({ expr1; expr2; ... ; exprN })

gives you something like a Lisp progn: it's an expression, whose value is exprN.

Complete sample:

  #include <stdio.h>
  
  int main(int argc, char **argv)
  {
    char *foo = (argc > 1) ? ({
                  printf("consequent\n");
                  "condition is true: I have arguments";
                }) : ({
                  printf("alternative\n");
                  "condition is false: I don't have arguments";
                });
    puts(foo);
  }

This is almost certainly because RMS is a Lisp hacker. I.e. why GCC uses Lisp notations internally and functions with "mapcar" in the name and such.

I discovered it by accident sometime in 1993. I wanted the value of the last expression in a braced block. Maybe it can forced into an expression if you just put parentheses on it? By golly, the intuition worked.

Mainly, it is used in writing macros. Speaking of which:

  #include <stdio.h>

  #define ifblock(expr) (expr) ? (
  #define elseblock ) : (
  #define endblock )

  int main(int argc, char **argv)
  {
    char *foo = ifblock (argc > 1) {
                  printf("consequent\n");
                  "condition is true: I have arguments";
                } elseblock {
                  printf("alternative\n");
                  "condition is false: I don't have arguments";
                } endblock;
    puts(foo);
  }

They got that from lisp.

But in lisp it's more than just a syntax trick that only works for a few predefined cases.

And Algol and others. It's a nice feature when available, certainly my preference. I find I can often write things more cleanly given an expression oriented languages than ones dominated by statements.

Kotlin, too!

Reply on news.ycombinator.com