Replacement in a condition /;

Questions
1

How do I make this work intuitively?

f[x_] := x^2 /; a>0

f[2] /. a->1

I want the answer to be 4 because 1>0 is True, but it does not work. It works if I make a_ an argument of f, but I don’t want to for various reasons.

2

This works, but it is awkward:

g[x_,a_]:=x^2 /; a>0

h[x_]:=g[x,a]

h[2]/.a->1

4

3

The approach you tried doesn’t work because f[2] is evaluated before the replacement rule is applied.

An approach like the following certainly works:

Module[{fResult},
 a = 1;
 fResult = f[2];
 a =.;
 fResult
 ]

But that’s probably too ornate for whatever you are doing given that you don’t want to define an f[x_,a_].

Another approach is to define f as

f[x_] := x^2

and then to execute

If[a > 0, f[2]] /. a -> 1

or

If[a > 0, f[2],alternative] /. a -> 1

I’m sure there are lots of approaches, but which one you might use depends on the particular problem that you are coding.

4

Here is idiomatically correct version of your code that works:

f[x_] := x^2 /; a > 0

Block[{a = 1}, f[2]]
5

Block is indeed the correct solution.

6

Why so complicated? How about simply:

f[x_] := If[a > 0, x^2, "whatever"]

this works exactly as intended:

a=.;
f[2] /. a->1

Yours Henrik

7

Note that that was one of my suggestions.