Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.soft-sys.math.mathematica > #16689

Re: Do we need a When function?

Show all headers | View raw

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

Back to comp.soft-sys.math.mathematica | Previous | Next — Previous in thread | Find similar

Thread

Re: Do we need a When function? Itai Seggev <itais@wolfram.com> - 2014-03-15 07:51 +0000
  Re: Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-03-17 06:27 +0000
  Re: Do we need a When function? Itai Seggev <itais@wolfram.com> - 2014-03-19 08:23 +0000
    Re: Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-03-23 09:01 +0000

csiph-web