Re: Function handles for nonexisting functions

[John W. Eaton]

On 22-Apr-2009, Judd Storrs wrote:

| On Mon, May 5, 2008 at 5:50 PM, Moritz Borgmann <address@hidden> wrote:
| 
| >  Is this common?  Are there other (reasonable) cases where it makes
| >> sense to create a handle to a function that is not visible, but will
| >> become visible later?
| >>
| >
| > I can't imagine any.
| >
| 
| For what it's worth,
| 
| It may be nice if octave didn't halt until it tried to actually evaluate an
| invalid handle. I'm not saying it should try and repair handles but it could
| keep invalid handles around as invalid.
| 
| For example, the SPM/niftilib Matlab code appears to use bogus handles as a
| way of marking un-implemented features. They use "@crash" entries to mark
| some corner cases in a dispatch table. This is easy to work around by
| creating a crash function that crashes ;), but I don't know how common this
| sort of thinking is.
| 
| In this case they used "@crash" but a matlab coder could concievably get
| really fancy and name the undefined functions such that the error hints at
| which specific feature wasn't implemented.

Will you please try the following with Matlab and tell me what
happens?

Create directories d1 and d2.  Put the following fucntion in
d1/crash.m:

  function crash ()
    'd1/crash'

and put the following function in d2/crash.m:

  function crash ()
    'd2/crash'

Now start Matlab in the directory that contains d1 and d2, and run the
following commands:

  fh = @crash
  fh ()
  cd d1
  fh ()
  cd ../d2
  fh ()

  clear all
  fh = @crash ()
  cd ..
  fh ()
  cd d1
  fh ()

Now, while Matlab is still running, change the definition of
d2/crash.m to be

  function crash ()
    'foo'

and then continue your Matlab session with the following commands:

  fh ()
  cd ..
  fh ()
  cd d2
  fh ()

  clear all
  addpath (pwd)
  cd ..
  fh = @crash
  fh ()

Now, while Matlab is still running, change the definition of
d2/crash.m to be

  function crash ()
    'bar'

and then continue your Matlab session with the following commands:

  fh ()
  cd d1
  fh ()
  cd ../d2
  fh ()

jwe