- To: mathgroup at smc.vnet.net
- Subject: [mg15966] Re: MyPrependTo
- From: "Allan Hayes" <hay at haystack.demon.co.uk>
- Date: Fri, 19 Feb 1999 03:26:54 -0500
- References: <firstname.lastname@example.org>
- Sender: owner-wri-mathgroup at wolfram.com
Will Self wrote in message <7ag26u$ag9 at smc.vnet.net>...
>In trying to create my own version of PrependTo, I ran into some strange
>stuff that I don't understand. I am calling my function "push". Please
>look at the session below and explain these mysterious happenings.
>It seems clear that there is more than one symbol e kicking around,
>but I can't make head or tail of it. See also my next message for
>the larger context.
>push[someStack_, elt_] := (someStack = Prepend[someStack, elt];)
>stack[2,1,"expression"] (* WHAT!!!!???? *)
>e = stack["expression"] (* DOUBLE WHAT!!!!???? *)
>push[someStack_, elt_] := (someStack = Prepend[someStack, elt];
push[s,e] causes the evaluation of value[s] = Prepend[s,e]
not the evaluation of s = Prepend[s,e]
simply adds the assignment stack[e] = stack[1,e]
so that we now have stored
s = stack[e]
stack[e] = stack[1,e]
(nowhere is e = stack[1,e] stored; the current value of e has to be
constructed from the two preceding definitions: e -> stack[e] ->stack[1,e])
Now: Rest[e] evauates as follows
Rest[e] -> Rest[stack[e]] -> Rest[stack[1,e]] -> stack[e] -> stack[1,e]
The last step is the surprise!
When ?e is entered it gives the value of e by using the current stored
definitions: this does not reveal the mechanism above.
Using FullDefinition[e] will reveal all.
Mathematica Training and Consulting
hay at haystack.demon.co.uk
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