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Re: HoldFirst, Unevaluated

• To: mathgroup at smc.vnet.net
• Subject: [mg109627] Re: HoldFirst, Unevaluated
• From: Albert Retey <awnl at gmx-topmail.de>
• Date: Sat, 8 May 2010 07:09:16 -0400 (EDT)
• References: <hs0pp2$dsn$1@smc.vnet.net>

Am 07.05.2010 12:24, schrieb Fred Klingener:
> Here, the second execution of f fails because variable 'a' evaluates
> to Pi and can't be reassigned:
> 
> ClearAll[f, a, b, c]
> f[x_, y_, z_] := x = Pi; 3 y + 2 z
> f[a, b, c]
> f[a, b, c]
> 
> I have come to understand that the canonical approach to control this
> is to assign the HoldFirst attribute to f:
> 
> ClearAll[f, a, b, c]
> SetAttributes[f, HoldFirst]
> f[x_, y_, z_] := x = Pi; 3 y + 2 z
> f[a, b, c]
> f[a, b, c]
> a
> 
> and, indeed, this works as intended.
> 
> Plan B, using an explicit Unevaluated also works:
> 
> ClearAll[f, a, b, c]
> f[x_, y_, z_] := x = Pi; 3 y + 2 z
> f[Unevaluated[a], b, c]
> f[Unevaluated[a], b, c]
> a
> 
> An alternate function formulation fails, evidently in the same way as
> the first example:
> 
> ClearAll[f, a, b, c]
> SetAttributes[f, HoldFirst]
> f = (#1 = Pi; 3 #2 + 2 #3) &
> f[a, b, c]
> f[a, b, c]
> a

You probably want this:

ClearAll[f, a, b, c]

f = Function[Null,#1 = Pi; 3 #2 + 2 #3,HoldFirst]


> 
> ClearAll[f, a, b, c]
> f = (#1 = Pi; 3 #2 + 2 #3) &
> f[Unevaluated[a], b, c]
> f[Unevaluated[a], b, c]
> a
> 
> does.
> 
> What am I missing?

Attributes of Symbols affect only the downvalues you have defined for
them, in the case that did not work, you did assign an ownvalue, which
happens to be a function. Thus the HoldFirt did not have an effect,
since the Function still did evaluate its arguments. You can set
attributes to Functions as shown above...

hth,

albert