Will Robertson writes:
 > On 07/09/2008, at 5:50 PM, Joseph Wright wrote:
 > 
 > > Only the last one works as intended (by me).  Is there a LaTeX3 way to
 > > do this?
 > 
 > Interesting.
 > This is weird:
 > 
 >    \err_warn:nn{\temp~stuff~stuff}{}
 > 
 > Only the second tilde causes a space.

why would that be weird?

in plain TeX

  \errwarn{\temp stuff stuff} 

and only

  \errwarn{\temp\space stuff stuff}

in expl3 ~ is a space token and not an active char expanding to \space.

It is a bitof a pity that space tokens are so deep down within the TeX engine
behavior and that sometimes they get gobbled if explicit.

Interesting question if it would be possible to do ~ -> \space instead but my
guess is that this would break a lot of the current code

frank