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Re: HoldFirst confusion


Jon Harrop wrote:
> Yaroslav Bulatov wrote:
> > The following gives an error
> > f = 5; f[a_] = a
> >
> > Set is HoldFirst, so why does it evaluate it's first argument f[a_]?
>
> Conversely, why do the attributes of Set and SetDelayed indicate that they
> hold their first argument when the kernel evaluates it.
>
> On a related note, how can you define and use a downvalue that matches
> g[1+2] without evaluating the 1+2?
>
> --
> Dr Jon D Harrop, Flying Frog Consultancy Ltd.
> http://www.ffconsultancy.com/products/ocaml_for_scientists/chapter1.html

Hello,

The point about HoldFirst is that it suppresses the automatic
evaluation of arguments before a function is called. It is easy to
write a function that has attribute HoldFirst but evaluates its
argument somewhere in its body - all that is necessary is to pass it on
to a deeper function that does not hold the argument!

Regarding the definition of g, the following seems to work:
SetAttributes[g,HoldFirst]

g[1+2]:=Print["xxx"]

David Bailey
http://www.dbaileyconsultancy.co.uk


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