Check-in [eb537ee186]

# TIP 450: Add [binary] subcommand "set" for in-place modification
	Author:         Arjen Markus <[email protected]>

	State:          Draft
	Type:           Project
	Vote:           Pending
	Created:        18-Jul-2016
	Post-History: 
	Tcl-Version:    8.7
	Keywords:       Tcl, binary data
................................................................................

The new command will have the effect that the first few steps are not
necessary anymore.

# Implementation Notes

Besides the nominal case of a variable that contains a binary array that is to
be updated within the bounds of that array, three other cases exist and need
to be prepared for:

 * The variable _varName_ does not exist yet


 * The variable _varName_ does not contain a binary array


 * The updating would go past the memory allocated for the binary array




Each of these cases and perhaps others will have to be taken care of. The
first case might be treated as if **binary format** was meant. For the
second case the implementation can convert the current value.



The third case might either cause an error \(we are updating an existing block
of memory after all\) or silently extend the memory, effectively performing
what the Tcl implementation shown above would do. If an error is thrown, then
the first case should probably throw an error as well.



# Reference Implementation

To be committed to a fossil branch.

A few remarks:

 * The third case is not completely treated yet \(see "TODO" in the code\)

 * The **binary set** command does not properly invalidate the string
   representation. The binary array is, however, updated properly - at least
   according to the very limited tests that were performed.

 * There are no proper test cases yet.

 * There is no proper performance measurement yet; this could be based
   on Wiki page <http://wiki.tcl.tk/44363.>

 * It does not deal with the possibility that the binary array is shared.

# Copyright

This document is placed in public domain.

# TIP 450: Add [binary] subcommand "set" for in-place modification
	Author:         Arjen Markus <[email protected]>
	Author:         Donal K. Fellows <[email protected]>
	State:          Draft
	Type:           Project
	Vote:           Pending
	Created:        18-Jul-2016
	Post-History: 
	Tcl-Version:    8.7
	Keywords:       Tcl, binary data
................................................................................

The new command will have the effect that the first few steps are not
necessary anymore.

# Implementation Notes

Besides the nominal case of a variable that contains a binary array that is to
be updated within the bounds of that array, four other cases exist and need
to be prepared for:

 * The variable _varName_ does not exist yet: we must create the
   variable when writing back the binary value.

 * The variable _varName_ does not contain a binary string: we must
   convert the variable to a binary string (using the usual algorithm).


 * The variable _varName_ contains a binary string that is shared with
   other uses (such as being stored in other variables): we must
   duplicate the binary string in this case before updating it.




 * The updating would go past the memory allocated for the binary
   array: we must extend the binary array.
   



_It is the Tcl programmer's responsibility to avoid these cases if they
matter to them._ The **binary set** command will act in accordance
with existing Tcl standard semantics.

# Reference Implementation



See the `tip-450` branch.














# Copyright

This document is placed in public domain.