[Ur] (1) Syntax errors when destructuring a record of records on a field value; (2) Is it ok to define a function which returns something of kind `Name`?

[Stefan Scott Alexander]

While attempting to extend the Crud demos, I need to do destructuring on a
record of records, eg on a value 'theCols' of type 'colsMeta(int, string)'
(slightly abbreviated below):

  val theCols : colsMeta(int, string)

  val theCols = [
    FooColumn = [
      Nam = "foo",
      Show = (fn s => s),
      Widget = (fn w => w) ],
    BarColumn = [
      Nam = "bar",
      Show = (fn s => s),
      Widget = (fn w => w) ]
  ]

In this case, I have a value of type `string` (eg, "bar") which corresponds
to the value of one of the fields of one of the "inner" records of
'theCols'.

Using just the string "bar" and the value 'theCols', I want to use
pattern-matching and destructuring, to do a kind of lookup on "bar" (to
find the field whose 'Nam' field = "bar"), and then assign two values:

  (a) myCol
  (b) myColNam

---------

So, given a record of records, eg:

  val theCols = [
    FooColumn = [
      Nam = "foo",
      Show = (fn s => s),
      Widget = (fn w => w) ],
    BarColumn = [
      Nam = "bar",
      Show = (fn s => sh),
      Widget = (fn w => w) ]
  ]

And given a *string* which equals one of the 'Nam' fields, eg:

  "bar"

The goal is to do the following two things:

(a) Assign 'myCol' to the entire field BarColumn (ie, the field name, and
the field value):

  val myCol =
    [ BarColumn = [
        Nam = "bar",
        Show = (fn s => s),
        Widget = (fn w => w) ]
    ]

(b) And assign 'myNam' to *just* the field name:

   val myColNam = #BarColumn

---------

I think the two functions below should do the trick:

  fun getCol aNam someCols =
    case someCols of
        { colNam = {
            Nam = aNam,
            Show = sh,
            Widget = wg },
          otherOuterFields } =>
        [ colNam = [
            Nam = aNam,
            Show = sh,
            Widget = wg ] ]
      | _ => error

  fun getNam aNam someCols =
    case someCols of
        { colNam = {
            Nam = aNam ,
            otherInnerFields },
          otherOuterfields } => #colNam
      | _ => error

  val myCol = getCol "bar" theCols

  val myColNam = getNam "bar" theCols

---------

Remark:

The patterns 'otherOuterFields' and 'otherInnerField' could probably be
replaced by wildcards `_`.

---------

Questions:

(1) I'm concerned about 'colNam', because it is of kind `Name`.

(a) Is it possible to have a function which returns something of kind
`Name`?

(b) Is it possible to say something like:

  val myColNam = getNam "bar" theCols

which attempts to assign something of kind `Name` to the identifier
'myColNam'?

(2) In function getCol, am I using { } and [ ] correctly - ie:

- { } for record *patterns*, and
- [ ] for record *expressions* (values) ?

---------

Syntax error:

So far, the above fragments (currently part of a larger body of code)
aren't parsing - I'm getting a syntax error on the 'of' in the 'case'
statement, saying "replacing  OF with  LPAREN".

I'm going to keep whittling my code down to make it smaller and smaller
until I can isolate the syntax error.

---------

Question:

Aside from the syntax errors, is the overall approach above approximately
correct?

---------

Comparison with the `as` keyword in ML:

Initially I though I could use something similar to ML's "layered
patterns", which use the keyword `as`.

Citations:

(1) "Tips for Computer Scientists on Standard ML (Revised)" by Mads Tofte,
page 6:

https://www.itu.dk/people/tofte/publ/tips.pdf

(2) "Elementary Standard ML", by G. Michaelson, section 6.9, page 138:

https://books.google.com.br/books?id=Zf0aq8BtSLAC&pg=PA138&lpg=PA138&dq=ML++%22layered+patterns%22&source=bl&ots=g2aDg-Ma6Q&sig=mAHDVFIRTWkqRAJt5zJj0nH7fvQ&hl=en&sa=X&redir_esc=y#v=onepage&q=ML%20%20%22layered%20patterns%22&f=false

But I don't think Ur/Web supports ML's "layered patterns" using the `as`
keyword, so the above functions 'getCol' and 'getNam' do the destructuring
using a "bigger" pattern.

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