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I answered “Equating matrices (or higher order tensors) element-wise” with:

{A, B} = RandomInteger[3, {2, 4, 3, 2}];

  SetAttributes[Equal, Listable];
  A == B
{{{False, False}, {False, True},  {False, False}}, 
 {{False, True},  {False, False}, {True, False}},
 {{False, True},  {False, False}, {False, False}}, 
 {{False, False}, {False, False}, {True, False}}}

But it fails if A and B are packed arrays:

{A, B} = Developer`ToPackedArray /@ {A, B};

  SetAttributes[Equal, Listable];
  A == B

One may observe that setting SetAttributes[Equal, Listable] outside of Block also fails to effect the Listable behavior. This also applies to Unequal and SameQ. This is of interest to me, but my prime concern is that despite the Block, Equal is being recognized and receiving special treatment. I expected symbols localized with Block to behave generically.

  • How does this fit into the main evaluation loop?

  • How can I get get true "blocking" of a symbol such that it behaves generically within a scoping construct?

  • What other symbols besides Equal, Unequal, and SameQ are handled differently?

share|improve this question
I think corner cases like this are to be expected for a high-level interpreted language which contains efficient data structures for special cases. You have to bypass the main evaluator as early as possible to be efficient, and at some point you have to make a choice which evaluation model to use. I am actually surprised that these corner cases are so few. A more consistent way to gain efficiency would probably be through making it possible to compile a larger subset of the language. – Leonid Shifrin Mar 20 '12 at 18:22
@Leonid I suppose then that the only thing "troubling" is my lack of knowledge of Mathematica. :-) – Mr.Wizard Mar 20 '12 at 22:52
Oh no, this is troubling, because this sort of explicitly violates the semantics of the language (albeit in one of its darker corners). I was just saying that it is hard to avoid such violations in a language with two distinct efficiency scales. Compiled languages solve this by having interpreted REPL but also compilation to native code. Some interpreted languages solve it by having JIT compilers in them. The solution currently used by Mathematica probably suits Mathematica as a system but is hardly adequate for it as a general-purpose programming language, from a purist's viewpoint. – Leonid Shifrin Mar 20 '12 at 23:26
Using Block[{Equal},SetAttributes[Equal, Listable];Equal[a_,b_]:=Hold[Equal[a,b]];A==B]//ReleaseHold gives {{{False, False}, {False, True}, {False, True}}, {{False, False}, {False, False}, {True, False}}, {{False, True}, {False, False}, {False, False}}, {{False, True}, {False, False}, {False, True}}} – celtschk Apr 10 '12 at 10:24
@celtschk using Trace on your expression v. Mr. Wizard's shows that by redefining Equal, as you do, it is actually threaded over the lists, yet Mr. Wizard's is not. Using SetSystemOptions["PackedArrayOptions" -> "UnpackMessage" -> True] reveals that in both cases some unpacking does occur, but in Mr. W's case we get "Unpacking array with dimensions {1}" v. "Unpacking array with dimensions {4,3,2} in call to Equal" in your case. By redefining Equal it automatically unpacks the full array. – rcollyer Apr 11 '12 at 17:21

you are good! Based on your comments, I crafted this:

Block[{Equal, H = Developer`ToPackedArray},
 SetAttributes[Equal, Listable];
 Equal[x_H, y_H] := 
  Equal[Developer`FromPackedArray[x], Developer`FromPackedArray[x]];
 a == b]

which works in both cases.


Mr.Wizard said " I expected symbols localized with Block to behave generically." Although the above works, as mentioned in the comments, there is an observable difference between Equal and other user defined symbols in the rewrite/eval loop in regards to Listable and automatic unpacking of packed arrays. I played with different definitions and couldn't find why this happens.

Clear[f, g, h, z1, z2];
ClearAttributes[{z1, z2}, {Listable}];
f[a_, b_] := Block[{Equal, H = Developer`ToPackedArray},
   SetAttributes[Equal, Listable];
   Equal[x_H, y_H] := 
   Print[ a == b  // FullForm];
   a == b];
g[a_, b_] := Block[{Equal, H = Developer`ToPackedArray},
   SetAttributes[Equal, Listable];
   Print[ a == b  // FullForm];
   a == b];
h[a_, b_] := Block[{Equal = z1, H = Developer`ToPackedArray},
    SetAttributes[z1, Listable];
    Print[ a == b  // FullForm];
    a == b] /. z1 -> Equal;
i[a_, b_] := Block[{Equal = z2, H = Developer`ToPackedArray},
    SetAttributes[Equal, Listable];
    Print[ a == b  // FullForm];
    a == b] /. z2 -> Equal;

Then I did

{a , b } = {{1, 2}, {1, 3}};
{c , d} = Developer`ToPackedArray /@ {a, b};

which produces,

In[211]:= f[a, b]


Out[211]= {True, False}

In[212]:= f[a, c]


Out[212]= {True, True}

In[213]:= g[a, b]


Out[213]= {True, False}

In[214]:= g[a, c]


Out[214]= True

In[215]:= h[a, b]


Out[215]= {True, False}

In[216]:= h[a, c]


Out[216]= {True, True}

In[217]:= i[a, b]


Out[217]= False

In[218]:= i[a, c]


Out[218]= True

So one can see that there is a difference between g[a,c] and h[a,c]: in g Equal does not unpack, whereas in h the user-defined z1 does. I think all the other behaviours can be explained from the evaluation (rewrite) steps as explained in the Mathematica documentation.

Anyway, just wanted to comment finally that although Mr Wizard's is a fair claim. There are a number of areas where other than pure symbolic/rewrite manipulation is occurring, and that simply Block is not probably considering. For example -hope not to trivial for you-, Block[{Equal}, ToExpression["?Equal"]] still prints the Equal documentation, instead of a reference to an undefined symbol. So, like in this case, maybe Equal (and other built-ins) have special behaviour which Block is not touching.

Sorry, I will leave it as answer, but now probably I should say it is not...

Update 2

Actually, just checked that Block leaves ::usage untouched! So, if you do

f::usage = "Symbol f";



Symbol f

And if you do Block still you get the same

Block[{f}, Information[f]]

Symbol f

so Block and Information are not working together, even for user-defined symbols!

Last update

As noticed by some, symbols like Plus have special behaviour too. So this

Block[{Plus}, Print[Trace[Plus[1, 2]]]]; (* 1 *)
Block[{Global`Plus}, Print[Trace[Plus[1, 2]]]]; (* 2 *)
Block[{Global`Plus}, Print[Trace[Global`Plus[1, 2]]]]; (* 3 *)





which demonstrates that Plus retains the built-in behaviour in 1 and 2, being really overridden only with a syntax like 3, which is not convenient. By the way, 2 generates a warning.

Bottom, line, to answer Mr.Wizard's request -I believe this answer was suggested in the comments- it seems the only generic way to override a built-in is to provide your own user defined symbol instead. If one wants to redefine, say Plus, IMHO it is not a burden as whatever definition one wants to introduce can be done with the user-defined symbol and still one has the convenience of the syntactic sugar. To wit

Block[{Plus = plus}, plus[0, _] := 0; Print[Trace[0 + 2]]];

which produces


So, I will leave it like this.

share|improve this answer
isn't this just another example of the observation from the comments - that any user definition will cause the array to be unpacked? For example using Equal[x_dummy]=Null will have the same effect – Simon Woods Dec 19 '12 at 20:31
I'm sorry, but I agree with Simon. I don't think this addresses the the heart of my question. – Mr.Wizard Dec 19 '12 at 21:44
I thought the point of your question was if Equal had special behaviour in Block, that's what I understood. I edited my answer to clarify this. Besides, I tried the code without the x_H case and it didn't work, so I disagree with the general unpacking behaviour mentioned. – carlosayam Dec 19 '12 at 22:12
Apologies, you guys were right @Mr.Wizard, there are observable differences between built-in symbols in Block as opposed to user defined. I wonder if there are more than the ones mentioned, or even if Block cannot fully clear a symbol in all areas of the Mathematica kernel - regardless if it is built-in or user-defined. – carlosayam Dec 20 '12 at 6:38
@Mr.Wizard, see Update 2! I bet there is a negative answer to your question "How can I get true blocking of a symbol such that it behaves generically within a scoping construct?" - there is not such thing, even for user-defined symbols. – carlosayam Dec 20 '12 at 7:42

Maybe it helps. It's another ways to copy and change internal functions attributes using Function. Here we create a function myEqual just as Equal but listable, so we don't have to use Block:

{A, B} = Developer`ToPackedArray /@ {A, B};
A ~myEqual~ B


Note that Null inside Function is just an undocumented property. If you get more comfortable you can just use: Function[{a,b},Equal[a,b],Listable];

You can make my equal local, just as you did in your example:

{A, B} = Developer`ToPackedArray /@ {A, B};
    A ~myEqual~ B


As @OleksanrR. commented. A clever way to define myEqual is:

Function[Null, Equal[##], Listable]

So we don't have to restricted to 2 arguments.

share|improve this answer
Why not use SlotSequence? (Indeed, that would seem to be the main reason to use this form of Function outside of playing tricks with argument renaming.) – Oleksandr R. Feb 24 '13 at 3:13
You are right. Updated. tks! – Murta Feb 24 '13 at 12:02

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