Why does SML not recognize certain types unless I put it in a variable?

Question

I have the following type:

datatype pattern = Wildcard
         | Variable of string
         | UnitP
         | ConstP of int
         | TupleP of pattern list
         | ConstructorP of string * pattern

I am trying to create a function that accepts this type (simplified example):

fun check_pat p =   
     case p of
        Wildcard          => 1
      | Variable x        => 2
      | TupleP ps         => 3
      | ConstructorP(_,p) => 4
      | _                 => 5

but get an error when I call it with TupleP like so:

check_pat TupleP[Variable "x",Variable "xx"];

Error:

stdIn:3.1-3.46 Error: operator and operand do not agree [tycon mismatch]
  operator domain: pattern
  operand:         pattern list -> pattern
  in expression:
    check_pat2 TupleP

But if I set this TupleP type to a variable and call it with that variable I am able to get the intended result. Could anyone let me know why this is the case?

 val x = TupleP[Variable "x",Variable "xx"];
 check_pat x;
 val it = 3 : int

SML interprets check_pat TupleP[Variable "x",Variable "xx"]; as (check_pat TupleP) [Variable "x",Variable "xx"]; which is obviously something you do not want. But check_pat (TupleP [Variable "x",Variable "xx"]); should work fine. — qouify
– qouify, Commented Nov 25, 2022 at 7:44
The error message basically says that TupleP is something that produces a pattern from a pattern list (i.e., pattern list -> pattern) whereas check_pat expects a pattern. — qouify
– qouify, Commented Nov 25, 2022 at 7:48
Also to enhance readability, I suggest you integrate pattern matching in your function definition instead of using case, i.e., write fun check_pat Wildcard = 1 | check_pat (Variable _) = 2 | .... — qouify
– qouify, Commented Nov 25, 2022 at 7:51

ruakh · Accepted Answer · 2022-12-03 23:26:45Z

3

As noted in comments, this:

check_pat TupleP[Variable "x",Variable "xx"]

parses as this:

(check_pat TupleP) [Variable "x",Variable "xx"]

rather than what you likely intended:

check_pat (TupleP [Variable "x",Variable "xx"])

Assigning TupleP [Variable "x",Variable "xx"] to x and then calling check_pat x also ensures that check_pat is called correctly.

edited Dec 3, 2022 at 23:26

ruakh

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answered Nov 25, 2022 at 15:42

Chris

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