Krdytkyu

Say after some long computation we get an expression

expr=x^2

We do not know what the value of expr beforehand.
Now we want to turn this in a function. We can use either

f[x_]=expr

or

f[x_]:=Evaluate[expr]

as suggested in this question.

However, when we do

f[x_]=Expand[expr]

or

f[x_]:=Evaluate[Expand[expr]]

The Expand will not have any effect.

Is there any way to make this work? Of course, we can define another function

g[x_]:=Expand[f[x]]

But is there any way to do it a bit more concisely?

Update:

If you do what suggested by Kuba in the comment, you get

In[89]:= With[{expr = expr}, f11[x_] := Expand[expr]]

In[90]:= f11[a + b]

Out[90]= x^2

In[91]:= ?? f11

Notebook$$34$907690`f11

f11[x$_]:=Expand[x^2]

Sorry, I was too hasty in comments:

With[{expr = expr}, SetDelayed @@ Hold[f[x_], Expand[expr]]]

SetDelayed @@ Hold is needed because of: Enforcing correct variable bindings and avoiding renamings for conflicting variables in nested scoping constructs

(f[x_] := Expand@#) &@expr

Dear @ablmf you can use the following syntax in Mathematica.

f[x_]:=x^2;

When you input any desired value for x, Mathematica gives you its value in f[x]. Say, you want f[2], Mathematica gives you 4

f[2]

4

You can also use it to obtain a list. Consider the following code

ClearAll["Global`*"];

f[x_] := x^2;



Table[

f[x]

, {x, 0, 10}]

it gives you

{0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100}