Odd edge-case behavior of Block
- To: mathgroup at smc.vnet.net
- Subject: [mg129955] Odd edge-case behavior of Block
- From: Roger Wilson <rogerhw999 at gmail.com>
- Date: Wed, 27 Feb 2013 23:45:14 -0500 (EST)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- Delivered-to: l-mathgroup@wolfram.com
- Delivered-to: mathgroup-newout@smc.vnet.net
- Delivered-to: mathgroup-newsend@smc.vnet.net
Block is used to localize the variable cache as used in the function g when called from the function f. In f the cache symbol to use is passed in as the second parameter z.
f[a_, z_] := Block[{cache = z}, g[a]];
g[a_] := (Print["hash" -> Hash[cache]]; cache[1] = a; cache[1]);
Everything works as expected...
Hash[z]
2065959314 (your number may be different)
Calling f...
f[1, z]
hash->2065959314 (same)
1
The symbol z now has a single downvalue...
DownValues[z]
{HoldPattern[z[1]] :> 1}
However what if I actually want to use the symbol cache as the cache symbol...
Hash[cache]
1028301578 (your number may be different)
Calling f...
f[1, cache]
hash->1028301578 (same)
1
Nothing stored in the cache this time?
DownValues[cache]
{}
My theory is that Block[{cache = cache},... effectively localizes cache to a local temporary variable but that doesn't explain why I see the right Hash for the symbol inside g.