Pattern-match empty Fifo.fifo in SML/NJ

When I try to write something like

fun test Fifo.empty = true
  | test _ = false`

I get an error message that says Error: variable found where constructor is required: Fifo.empty. I'm really new to SML/NJ. Turns out it has something to do with Fifo.empty being a longId, but I haven't figured out how to fix this, except by patching it by passing the Fifo.isEmpty as an argument, but that's hardly a solution...

Solution

When you inspect the Fifo module,

- open Fifo;
[autoloading]
[library $SMLNJ-LIB/Util/smlnj-lib.cm is stable]
[autoloading done]
opening Fifo
  datatype 'a fifo = ...
  exception Dequeue
  val empty : 'a fifo
  (* and so on *)

you can see that Fifo.empty is an 'a fifo value. To see how it's made,

- Fifo.empty;
val it = Q {front=[],rear=[]} : 'a fifo

Unfortunately the Q datatype constructor of 'a fifo is hidden by the module being opaque (so the datatype definition appears as "...". When you define a datatype, its constructors (e.g. Q) become both value constructors and pattern constructors, but when you declare values like Fifo.empty using such value constructors, they don't also become pattern constructors.

I haven't figured out how to fix this, except by patching it by passing the Fifo.isEmpty as an argument, but that's hardly a solution...

I'm not sure why you would need to pass in Fifo.isEmpty as an argument; can't you just refer to Fifo.isEmpty in the function body? Perhaps there are dimensions to this problem you're not explaining.

How about the following:

fun test queue = Fifo.isEmpty queue

Or simply:

val test = Fifo.isEmpty

Generally, if you feel like passing in a bunch of library functions as arguments, you could instead consider building a higher-order module (functor) that takes another module as argument.