In 'from' clause, allow 'variable = value'

[julianhyde]

In 'from' clause, allow 'variable = value'. For example,

from e in hr.emps,
 fullName = e.firstName ^ " " ^ e.lastName
yield {e.empno, fullName};

It is syntactic sugar for iterating over a singleton collection:

from e in hr.emps,
 fullName in [e.firstName ^ " " ^ e.lastName]
yield {e.empno, fullName};

But I think the syntax may be useful for users.

In this case you could rewrite to a let:

from e in hr.emps
yield
  let
    val fullName = e.firstName ^ " " ^ e.lastName
  in
    {e.empno, fullName}
  end;

But this would not be possible if fullName were used in a later in or a where clause.

Further, now that #62 has implement the function only: α list → α (which throws if the list is empty or has more than one element), we can write

from e in hr.emps,
  d = only (from d in hr.depts where d.deptno = e.deptno)
yield
  {e, d};

This is important in asserting that a sub-query returns precisely one row. The same effect as a scalar sub-query in SQL:

SELECT e.*,
  (SELECT d.name
    FROM hr.depts AS d
    WHERE d.deptno = e.deptno) AS dname
FROM hr.emps AS e

(SQL is a little more limited, because you are only allowed to return 1 column.)

[julianhyde]

Note the similarity with the 'let' clause in XQuery ('let' is the 'L' in 'FLWOR'). Here is example 6-6 in XQuery by Priscilla Walmsley:

let $doc := doc("catalog.xml")
for $prod in $doc//product
let $prodDept := $prod/@dept
let $prodName := $prod/name
where $prodDept = "ACC" or $prodDept = "WMN"
return $prodName

In Morel, I think it makes sense to intermingle 'in' and '=' in the same from expression. In XQuery if you want a for followed by a let followed by a for you have to create a new query (for), which if nothing else increases the indentation.

[julianhyde]

Fixed in 7fc889a.