Pattern matching compile failure

[Mike Jones]

  This code compiles with an error: this pattern match clause is redundant

But this code is like the example in the book. What led to this code was when the second case "idle" was missing, things behaved like the the first case "transacting" evaluated true no matter what s.ps was. So I added the second case just to debug that, and got the compile error, which kind of confirms it is always matching.

fun isWeekday

  (x: wday): bool = case x of
  | Saturday() => false | Sunday() => false | _ => true
// end of [isWeekday]

CODE

datatype pstate =

    | Idle of ()

    | Transacting of ()

    | Hang of ()

  typedef state = @{ addr= uint8, ps= pstate }

fun process (c:int, s:state) : state = let

    val  sn  = case 0 of

              | _ when ch = 'p' => let

                val i:int = case s.ps of

                  | transacting => let

                     val _ = i2c_stop()

                     in 1 end

                  | idle => let

                     val _ = i2c_start()

                     in 2 end

                val () = print! i

                in @{ addr = s.addr, ps = idle } end  

[Hongwei Xi]

If a nullary constructor Foo is used as a pattern, it needs to be

written as 'Foo()'; 'Foo' is considered a variable when used as a pattern.

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[Mike Jones]

I will store that one next to:

if (i = j)

{

  be_subtle_ and_cause_trouble();

}

Solved problem...

Thanks

[gmhwxi]

Unlike using '=' for '==' in C, using 'Foo' for 'Foo()' causes no trouble:

1) If 'Foo' is the pattern for the last clause, then 'Foo()' and 'Foo' are
    really the same (assuming pattern matching is exhaustive)
2) If 'Foo' is not the last one, then you always get a compilation error.