Need a type conversion mechanism to convert between strings and numbers

Syntax

{$toInt: <arbitrary expression>}

{$toLong: <arbitrary expression>}

{$toDouble: <arbitrary expression>}

{$toDecimal: <arbitrary expression>}

{$toString: <arbitrary expression>}

Examples

> db.coll.insert([

  {_id: 0, x: "3.4"},

  {_id: 1, x: 0.99}

]);

> db.coll.aggregate([{

  $project: {

    intField: {$toInt: "$x"}

  }

}])

{_id: 0, intField: 3}

{_id: 1, intField: 0}

// Example 2

> db.coll.insert([

  {_id: 0, x: {a: "b"}},

  {_id: 1, x: 1.22},

  {_id: 2, x: "abc"}

]);

> db.coll.aggregate([{

  $project: {

    stringField: {$toString: "$x"}

  }

}])

{_id: 0, stringField: "{a: 'b'}"}

{_id: 1, stringField: "1.22"}

{_id: 2, stringField: "abc"}

Notes

• Each numeric conversion can convert from any numeric type or from a string (truncating if necessary).
• Calling $toInt on an int is a no-op, similarly for other conversions.

Errors

• If the input is of a type or value that cannot be converted:
  $toInt, $toLong, $toDouble, $toDecimal accept only int, long, double, decimal, and strings that can be interpreted as the above. $toString accepts anything that can be turned into output that is parseable by the mongo shell.

Old Description
There are situations where I want to use a number as (part of) a string but $concat will only take strings and not numbers. And there are other times where I have a string of a number "2012" and I want to use it as a number and there is no operator to do this.

Seems painful to fall back to map/reduce solely because someone saves a number or date as a string