# Substituting within a function definition that includes Simplify

I would like to define a function like

myfunc[f_] := Simplify[(f/g)]


where g will be substituted/replaced with the current value of g at the time of definition. In practise, g will be complicated and myfunc will be defined over and over again. However, between these redefinitions I want it to be statically set. If

g=1+x^2


then I want the equivalent of typing

myfunc[f_] := Simplify[(f/(1+x^2))]


The following works but seems pretty arcane:

myfuncA[f_] := Evaluate[With[{yy = g}, Hold[Simplify[f/yy]]]]

myfunc[f_] := ReleaseHold[myfuncA[f]]


How to accomplish this simple task?

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• myfunc[f_] := Simplify[(f/g)] /. (g -> 1 + x^2) – Peter Roberge Apr 25 '16 at 17:43
• If the substitution for g were to be made for one particular instance and the value substituted was simple then this construction would work. In my example, the 1+x^2 is a placeholder for a complicated expression calculated elsewhere over and over again. So this particular method doesn't provide the flexibility I need. – user39757 Apr 26 '16 at 1:34

I think that value injection using With will work for you.

Let's first define the sample g value you want:

g = 1 + x^2


Now we define myfunc injecting the current definition of g inside its definition:

Clear[myfunc]
With[{g = g}, myfunc[f_] := Simplify[f/g]]


Now let's change the value of g and check whether the definition of myfunc is affected:

g = 3;

myfunc[expr]
(* Out: expr/(1+x^2) *)

myfunc[1 + x^2 - x^3 - x^5]
(* Out: 1 - x^3 *)

• This works and is the prototypical construction needed! – user39757 Apr 26 '16 at 12:30

Maybe something like this?

g = 1 + x^2
Unprotect[saveg]
saveg = g;
myfunc[f_] := Simplify[(f/saveg)]
Protect[saveg]