On 15.03.2006, at 16:55, Raphael Collet wrote:
For control structures, macros are often good enough.
I agree. Paul Graham [chap. 8, 1993] details the cases when a macro
should be defined instead of a function
(www.paulgraham.com/onlisptext.html). For example, control structures
which potentially don't evaluate their arguments (such as 'unless') can
not be defined otherwise. Alternatively, you may define 'unless' as a
first-class function, but its less convenient to always wrap the
consequence in a function/procedure.
One essential property of macros is that their expansion must be quite
obvious. Maybe recursive macros aren't that good, after all.
For something like 'while' you could do the actual work of the control
structure (such as recursive calls) in a higher-order procedure. For
convenience, a macro could then call this higher-order procedure and
wrap the actions in a procedure.
proc {While Cond Body}
if {Cond} then {Body} {While Cond Body} end
end
defmacro while Cond do Body end
{While proc {$} Cond end proc {$} Body end}
end
Best,
Torsten
--
Torsten Anders
Sonic Arts Research Centre
Queen's University Belfast (UK)
www.torsten-anders.de
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