Mathematica Module versus With or Block - Guideline, rule of thumb for usage?

Question

Leonid wrote in chapter iv of his book : "... Module, Block and With. These constructs are explained in detail in Mathematica Book and Mathematica Help, so I will say just a few words about them here. ..."

From what I have read ( been able to find ) I am still in the dark. For packaged functions I ( simply ) use Module, because it works and I know the construct. It may not be the best choice though. It is not entirely clear to me ( from the documentation ) when, where or why to use With ( or Block ).

Question. Is there a rule of thumb / guideline on when to use Module, With or Block ( for functions in packages )? Are there limitations compared to Module? The docs say that With is faster. I want to be able to defend my =choice= for Module ( or another construct ).

Answer 1

A more practical difference between Block and Module can be seen here:

Module[{x}, x]
Block[{x}, x]
(*
-> x$1979
   x
*)

So if you wish to return eg x, you can use Block. For instance,

Plot[D[Sin[x], x], {x, 0, 10}]

does not work; to make it work, one could use

Plot[Block[{x}, D[Sin[x], x]], {x, 0, 10}]

(of course this is not ideal, it is simply an example).

Another use is something like Block[{$RecursionLimit = 1000},...], which temporarily changes $RecursionLimit (Module would not have worked as it renames $RecursionLimit).

One can also use Block to block evaluation of something, eg

Block[{Sin}, Sin[.5]] // Trace
(*
-> {Block[{Sin},Sin[0.5]],Sin[0.5],0.479426}
*)

ie, it returns Sin[0.5] which is only evaluated after the Block has finished executing. This is because Sin inside the Block is just a symbol, rather than the sine function. You could even do something like

Block[{Sin = Cos[#/4] &}, Sin[Pi]]
(*
-> 1/Sqrt[2]
*)

(use Trace to see how it works). So you can use Block to locally redefine built-in functions, too:

Block[{Plus = Times}, 3 + 2]
(*
-> 6
*)

Answer 2

As you mentioned there are many things to consider and a detailed discussion is possible. But here are some rules of thumb that I apply the majority of the time:

Module[{x}, ...] is the safest and may be needed if either

1. There are existing definitions for x that you want to avoid breaking during the evaluation of the Module, or

2. There is existing code that relies on x being undefined (for example code like Integrate[..., x]).

Module is also the only choice for creating and returning a new symbol. In particular, Module is sometimes needed in advanced Dynamic programming for this reason.

If you are confident there aren't important existing definitions for x or any code relying on it being undefined, then Block[{x}, ...] is often faster. (Note that, in a project entirely coded by you, being confident of these conditions is a reasonable "encapsulation" standard that you may wish to enforce anyway, and so Block is often a sound choice in these situations.)

With[{x = ...}, expr] is the only scoping construct that injects the value of x inside Hold[...]. This is useful and important. With can be either faster or slower than Block depending on expr and the particular evaluation path that is taken. With is less flexible, however, since you can't change the definition of x inside expr.

Answer 3

Andrew has already provided a very comprehensive answer. I would just summarize by noting that Module is for defining local variables that can be redefined within the scope of a function definition, while With is for defining local constants, which can't be. You also can't define a local constant based on the definition of another local constant you have set up in the same With statement, or have multiple symbols on the LHS of a definition. That is, the following does not work.

With[{{a,b}= OptionValue /@ {opt1,opt2} }, ...]

I tend to set up complicated function definitions with Module enclosing a With. I set up all the local constants I can first inside the With, e.g. the Length of the data passed to the function, if I need that, then other local variables as needed. The reason is that With is a little faster of you genuinely do have constants not variables.

Answer 4

I'd like to mention the official documentation on the difference between Block and Module is available at http://reference.wolfram.com/mathematica/tutorial/BlocksComparedWithModules.html.