Definine a function which accepts either functions or constants as arguments

Question

I wish to define a function, which accepts either functions or constants as arguments. I tried

g[t_, y_] := Module[{}, t[y]]

which simply evaluates some function t at y.

Now if I run

t[x_] = 300
g[t, y]

the module g simply returns 300. However if I run

g[300, y]

it returns:

300[y] 

How do I define g in such a way that it accepts either functions or constants as arguments?

Answer 1

Edited

I'm uncomfortable with the idea of writing a function with the kind of special behavior you propose, but if I had to, I would implement it this way.

g[x_?NumericQ, y_] := x
g[t_, y_] := t[y]

Then

Clear[t]; t[x_] = 300; g[t, y]

300

Clear[h]; h[x_] := 3 x; g[h, y]

3 y

g[300, y]

300

g[#^2 &, y]

y^2

Answer 2

SetAttributes[g, Orderless]
g[t_Symbol, y_] := Module[{}, t[y]]

g[t, 200]

t[200]

g[300, y]

y[300]

t[x_] := 300
g[300, t]

300

g[t, 200]

300