[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]

[Adam Chlipala]

On 07/27/2015 07:50 PM, Stefan Scott Alexander wrote:
> Strangely, this error message mentions a field [Id = string] - but my 
> code only uses [Id = int].

The problem is in this code, where you use [r.Id] as an <option> value, 
which must be a string.  The type of [option] in basis.urs makes that 
requirement explicit.
>   colorOptions <-
>     queryX1 (SELECT * FROM color)
>     (fn r => <xml> <option value={r.Id}> {[r.Nam]} </option> </xml>);

> 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).

As the message indicates, you're supposed to do a text search for 
"<UNIF:" to see where the undetermined unification variables are.
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