Control structures in APL 
Author Message
 Control structures in APL

Further comments on Kim Whitelaw's proposal:

There are many easy ways to simulate most of Kim's proposal.  For example,
to get rid of the '' ugliness one can use a single backquote ` or a doublequote
" inside a code string.  To mark a string as code one can insert matched {} at
the beginning and end.  Nested blocks simply use nested {}.  A user-defined
execute can preprocess this string.  Multi-line blocks can be written using
easily defined functions

  'F' IS '...'
     AND '...'

which either join them with "diamond" or create a nested structure to be passed

using a dummy name, or preprocessed away.  Local functions can also be defined
using a trailer

     END 'F'

  'G' CALLS 'F'

and have CALLS check their fix times and (re)fix F if G is more current.
It works okay.  My point is that these techniques have been around a long
time, and people already use them, so I don't think Kim's proposal will
necessarily add much if any new structure to APL.  Another way to look at it
is that APL already has this kind of dynamically scoped local/"first class"
functions--namely the string.

Regarding the difficulty of implementing lexical scope, I want to point out
that a _tree_ of environments is only necessary for the so-called "upward
funarg", i.e. a function that is created and then passed _out_ of its
definition enviroment, which dies (exited) while the function remains dangling;
otherwise it is still a stack.  By keeping local functions local, this problem
does not arise.

Mon, 24 Feb 1997 06:05:10 GMT  
 [ 1 post ] 

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