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Re: Add syntax highlighting to own command

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  • Subject: [mg101899] Re: Add syntax highlighting to own command
  • From: earthnut at (Bastian Erdnuess)
  • Date: Tue, 21 Jul 2009 03:52:53 -0400 (EDT)
  • References: <> <h3766u$f9h$> <h41f80$rkq$>

Leonid Shifrin <lshifr at> wrote:

> Hi Bastian,
> If I understand you correctly and the last rule you added
> ( Let /: ( lhs_ := Let[ vars_, expr_ /; cond_ ] ) :=
>      Let[ vars, lhs := expr /; cond ])
> is to allow shared local variables in function definitions (like with
> standard With, Module or Block), then your code does not work for me.

Yes, you are (and understood me) right.

> Consider the following definition:

> [...]

> f[x_, y_] :=
>   Let[{xl = x, yl = y + xl + 1}, xl^2 + yl^2 /; (xl + yl < 15)];

I tried first something different than I actually posted.  I thought
then I could write it different.  But somehow I must have forgotten to
test it correctly.

However, also with my original version (something like

  f_[ l_, Let[ r_ ] ] := bla[ l := r ] /; f == SetDelayed  )

I would have gotten about

  f[ x$_, y$_ ] := x^2 + (1 + x + y)^2 /; 1 + 2x + y < 15  .

I didn't recognized that the local variables in f get renamed.

> I can think of  several reasons why this does not work, and will expand this
> discussion if  you will be interested, but here is the version that I
> believe does work:

Now, I can too.  But if you want to explain some of your reasons, i'll

I also found a trick to get around the renaming problem (using your
Verbatim instead of mine f_[...] /; f == ...):

  Verbatim[ SetDelayed ][ l_,
    Let[ v_, Verbatim[ Condition ][ r_, c_ ] ]
  ] ^:=
    Let[ v,
      SetDelayed[ #, r /; c ] & @ ToExpression @ #
    ] & @ ToString[ l, InputForm ]

This passes basically the left hand side as string around the issue with
the renaming.

The last days I often figured out that that what I think of as "syntax
transformation" or Simon called "passing unevaluated arguments around"
works best by passing the things around as strings.  Are there
disadvantages doing that?  I could imagine that this might be not the
best if one wants to compile the source -- but so far I'm far away from
starting to try to understand Mathematicas compiler.

> [ Block trick ]

> This uses the Block trick. It is actually a true macro, since
> it expands the code before SetDelayed acts on it:

That seems to be a nice trick.  Thank you for showing that.  I need to
think a little bit about that to decide if I like it for this situation.


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