I have come across the (internal) use of the function:


I am trying to determine its purpose. It seem to behave like Internal`InheritedBlock except that a starting value (e.g. {x = 3}) cannot be set.

x = "global"; f[] := x

Internal`LocalizedBlock[{x}, {x, x = 7, f[], Hold[x]}]

Internal`InheritedBlock[{x}, {x, x = 7, f[], Hold[x]}]
{"global", 7, 7, Hold[x]}


{"global", 7, 7, Hold[x]}


What purpose does this function serve? Why would it be used in place of InheritedBlock?


2 Answers 2


Internal`LocalizedBlock behaves the same as Block, but it can localize non-Symbols (e.g. f[1], Subscript[x, 0], etc.).

For example,

Internal`LocalizedBlock[{Subscript[x, 0]}, Subscript[x, 0] = 1]
(* 1 *)

Compare this to

Block[{Subscript[x, 0]}, Subscript[x, 0] = 1]
(* During evaluation of In[79]:= Block::lvsym: Local variable specification {Subscript[x, 0]} contains Subscript[x, 0], which is not a symbol or an assignment to a symbol. >> *)
(* Block[{Subscript[x, 0]}, Subscript[x, 0] = 1] *)

It's also worth noting that one cannot assign values in the first argument of Internal`LocalizedBlock

  • $\begingroup$ So would Internal`LocalizedBlock be an answer to this question: mathematica.stackexchange.com/questions/64673/… $\endgroup$
    – QuantumDot
    Jan 16, 2015 at 20:37
  • $\begingroup$ @QuantumDot It seems so. Perhaps you wish to self-answer? $\endgroup$
    – Mr.Wizard
    Jan 16, 2015 at 20:41
  • $\begingroup$ @Mr.Wizard I guess I'm not familiar enough with Inherited`LocalizedBlock to provide a confident answer. Maybe I'll add it in the comments section. $\endgroup$
    – QuantumDot
    Jan 16, 2015 at 22:29
  • 9
    $\begingroup$ @Mr.Wizard just a little tidbit, if I recall correctly this was added for the purpose of fitted models being able to work with subscripted variables so I think Dan is right on here. $\endgroup$
    – Andy Ross
    Jan 17, 2015 at 5:55
  • $\begingroup$ @Andy Thanks for the note. $\endgroup$
    – Mr.Wizard
    Jan 17, 2015 at 14:46

Disclaimer: these are all guesses, believe at your own risk.

As the first argument, you specify a list of patterns to localize. For example, {x}, {x[2]}, {x[][2]}, or x[2]/;True

This creates a dynamic environment around the second argument inside which the following happens:

  • Changes to those patterns' own/down/subvalues while in the environment aren't kept after its execution.
  • If the pattern is

    • a symbol (e.g x, f):

      works like block, and all of the symbol's definitions don't apply inside the localized block

    • a non-pattern pattern that doesn't have a downvalue/subvalue outside the environment: (by non-pattern pattern I just mean a pattern that don't have conditions, pattern tests, blanks, etc. such as x[2]] or x[][5]. I wonder if there's a standard name in the community for these)

      does nothing

    • any other pattern patt:

      Temporarily set the definition patt:=System`Private`$Localized. This is a definition that makes the whole downvalue/subvalue lookup fail to find a match. Here is a usage example;

Low importance note: this changes seem to be applied in order of appearance, so if the first argument is {x, x[2]/;True}, then the definition x[2]/;True:=SystemPrivate$Localized will be added, while it won't be with the order reversed to x[2]/;True, x}


You can use it to

  • Change specific down/sub/ownvalues of a symbol while in a dynamic environment without it affecting the outside, but allowing all other definitions changes to leak.
  • Work with the outside definitions of the symbol but making some particular pattern behave as undefined.
  • $\begingroup$ Is this how you propose Internal`LocalizedBlock works or simply an idea you wish to implement some time? $\endgroup$
    – Mr.Wizard
    Jan 16, 2015 at 18:47
  • $\begingroup$ @Mr.Wizard how I think it works. Does it fail easy tests? $\endgroup$
    – Rojo
    Jan 16, 2015 at 18:47
  • $\begingroup$ No, I just wasn't sure of your intent. :-) $\endgroup$
    – Mr.Wizard
    Jan 16, 2015 at 18:48
  • $\begingroup$ Ok, I now can test this. Let's see $\endgroup$
    – Rojo
    Jan 16, 2015 at 19:15
  • 2
    $\begingroup$ believe it or not $\endgroup$ Jan 16, 2015 at 20:42

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