[Ur] Minimal example of <select> widget for foreign-key field; error "Some constructor unification variables are undetermined" mentions field [Id = string] - but my field is actually [Id = int]

Mon Jul 27 19:50:51 EDT 2015

In order to minimize possible errors, I am trying to do minimal example of
a `<select>` widget to edit a foreign-key field, but I am still getting a
compile error "Some constructor unification variables are undetermined".

Strangely, this error message mentions a field [Id = string] - but my code
only uses [Id = int].

The code is on GitHub (and also reproduced at the end of this message).
It's very short and simple:

https://github.com/StefanScott/urweb-colorThing_Fk

It uses only two tables ("parent" table 'color', and "child" table 'thing'):

```
table color : {
  Id : int,
  Nam : string
}
PRIMARY KEY Id

table thing : {
  Id : int,
  Nam : string,
  Color : int
}
PRIMARY KEY Id,
CONSTRAINT Thing_isof_Color FOREIGN KEY Color REFERENCES color(Id)
```

It also makes the following additional simplifications:

- no sequence or auto-increment (ie, the user has to create the primary key)

- no meta-programming

- no Ur/Web modules, structures, functors or libraries

- web interface provided only for *creating* records in table 'thing' (no
updating or deleting)

- no web interface provided for creating, updating or deleting 'color'
records (must do this via via psql)

The Ur/Web compiler gives the following error:

```
Some constructor unification variables are undetermined in declaration
(look for them as "<UNIF:...>")
```

It can be difficult to debug this error, as it refers to a wide range of
lines (covering all the function definitions).

---------

Clue:

There is one interesting clue in the compiler error message: it frequently
mentions a field:

```
Id = string
```

However, both instances of 'Id' defined in the program (the primary keys of
tables 'thing' and 'color') have type 'int', not 'string'.

I'm very puzzled. Can anyone help explain this compile error?

The compile output is reproduced in Issue 1 of the GitHub repo:

https://github.com/StefanScott/urweb-colorThing_Fk/issues/1

---------

Below is the complete source code of colorThing_Fk.ur (also on GitHub).
It's very short, in order to make it easier find the error!

https://github.com/StefanScott/urweb-colorThing_Fk

table color : {
  Id : int,
  Nam : string
}
  PRIMARY KEY Id,
  CONSTRAINT Nam UNIQUE Nam,
  CONSTRAINT Id CHECK Id >= 0

table thing : {
  Id : int,
  Nam : string,
  Color : int
}
  PRIMARY KEY Id,
  CONSTRAINT Id CHECK Id >= 0,
  CONSTRAINT Thing_isof_Color FOREIGN KEY Color REFERENCES color(Id)

fun list () =

  thingRows <-
    queryX1 (SELECT * FROM thing)
    (fn r => <xml> <tr>
               <td> {[r.Id]} </td>
               <td> {[r.Nam]} </td>
               <td> {[r.Color]} </td>
             </tr> </xml>);
  colorOptions <-
    queryX1 (SELECT * FROM color)
    (fn r => <xml> <option value={r.Id}> {[r.Nam]} </option> </xml>);
  return
    <xml>
      <table border={1}>
        <tr> <th>Id</th> <th>Nam</th> <th>Color</th> </tr>
        {thingRows}
      </table>
      <br/><hr/><br/>
      <form>
        <table>
          <tr> <th>A:</th> <td><textbox{#Id}/></td> </tr>
          <tr> <th>B:</th> <td><textbox{#Nam}/></td> </tr>
          <tr> <th>C:</th> <td><select{#Color}>{colorOptions}</select></td>
</tr>
          <tr> <th/> <td><submit action={create} value="Add Row"/></td>
</tr>
        </table>
      </form>
    </xml>

and create r  =
  dml (INSERT INTO thing (Id, Nam, Color)
       VALUES ({[readError r.Id]}, {[r.Nam]}, {[readError r.Color]}));
  ls <- list ();
  return
    <xml><body>
      <p>Thing inserted!</p>
      {ls}
    </body></xml>

and main () =
  ls <- list ();
  return <xml><body>
    <h1>Things</h1>
    {ls}
  </body></xml>

---------

Thanks for any help!
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