Mathematica: set default value for argument to nonconstant?

Question

Can I set the default value for a function argument to be something that's not constant? Example:

tod := Mod[AbsoluteTime[], 86400] 
f[x_:tod] := x    

In the above, 'tod' changes every time I evaluate it, but "f[]" does not. "?f" yields:

f[x_:42054.435657`11.376386798562935] := x 

showing the default value was hardcoded when I created the function.

Is there a workaround here?

Answer 1

It seems to work if the function holds its arguments:

tod := Mod[AbsoluteTime[], 86400]
SetAttributes[f, HoldAll];
f[x_: tod] := x

In[23]:= f[]

Out[23]= 47628.994048

In[24]:= f[]

Out[24]= 47629.048193

Or you can use a construction like the following instead of a default value:

g[] := g[Mod[AbsoluteTime[], 86400]]
g[x_] := x

In[27]:= g[]

Out[27]= 47706.496195

In[28]:= g[]

Out[28]= 47707.842012

Answer 2

I recommend this:

f[] := f[Mod[AbsoluteTime[], 86400]]
f[x_] := x

Or equivalently, this:

f[x_:Null] := With[{x0 = If[x===Null, Mod[AbsoluteTime[], 86400], x]},
  x0]