
Re: Do we need a When function?
Posted:
Mar 23, 2014 5:01 AM


On 19/03/2014 08:23, Itai Seggev wrote: > David, > > Firstly, I would appreciate it if you didn't quote these tiny fragments of my > replies, which suggest I didn't already understand/address points you raise > (or that perhaps you didn't bother to read my email in its entirety). > >> On 15/03/2014 07:51, Itai Seggev wrote: >>> If[NumericQ[x], f[x]] (*are NumberQ, if want an actual number type*) >> >> The difference, which at least Szabolcs appreciates, can be seen in this >> example: >> >> In[3]:= expr = If[NumericQ[x], ToString[x]] >> >> In[4]:= expr /. x > 2.1 >> >> (* The output here is Null *) >> >> In[7]:= SetAttributes[When, HoldAll]; >> When[cond_, val_] := val /; cond >> >> In[9]:= expr = When[NumericQ[x], ToString[x]] >> >> Out[9]= When[NumericQ[x], ToString[x]] >> >> In[10]:= expr /. x > 2.1 >> >> Out[10]= "2.1" >> >> The effect of When is to delay its evaluation until something is true  >> which can be quite useful! > > But on the other hand you have these behaviors: > > In[59]:= ClearAll[When] > SetAttributes[When, HoldAllComplete]; > When[cond_, val_] := val /; cond > > In[62]:= expr = When[NumericQ[x], ToString[x]] > > Out[62]= When[NumericQ[x], ToString[x]] > > In[64]:= Block[{x = 2.1}, expr ] > > Out[64]= When[NumericQ[x], ToString[x]] > > In[65]:= x = 2.1; expr > > Out[65]= When[NumericQ[x], ToString[x]] > > Are these useful? > > > They are not useful  but I didn't define When with Attribute HoldAllComplete, but with HoldAll  so I don't see your point!
David Bailey http://www.dbaileyconsultancy.co.uk

