Re: datatype constructor as syntax

As  you guessed, constructors of datatypes are not just values, they are
syntactically special because they mean something special in a pattern.

Note, it is possible (although awkward and  a  kludge)  to  rebind  name
which  is  a  constructor  to a value.  The trick is that in a `val rec'
binding, the variable being bound position is NOT a pattern.  Of  course
the  right  hand  side  of  a val rec must be a function, so you have to
first rebind it to a function and then bind it again to  the  value  you
want.

Hence, the following is legal:

    datatype mytype = foo | bar of int

    val rec bar = fn () => ()

    val bar = 23

Of  course  this would mean that you could not use bar (the constructor)
in the right-hand side of the value that you want bar (the value)  bound
to.  To handle that case, you have to use some other variable name:

    local
       val tmp = <arbitrary expression>
       rec bar = fn () => ()
    in
       val bar = tmp
    end

where <tmp> is any variable name which is NOT a constructor in the outer
scope.

As I said, definitely a kludge.

As far as context-freeness, note  pretty  much  all  languages  are  NOT
context  free.   A  variable that is used has to be bound somewhere, and
that is not a context-free condition.

I don't understand what you mean by
    ... this kind of pattern matching is inherently non-exhaustive.

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