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I'm not sure if this problem has been asked before as I don't know the keyword to search for it.
I have a function where return result contains variable b (just for illustration).

myfunction[a_] := Module[{},
  1 + (2 a - 1) b ]

I want it to always return an expression with b unevaluated and only evaluate it when I do some replacement.

enter image description here

enter image description here

This one works as b is not used anywhere (not being assigned some value).
However, now assume that b is assigned somewhere then the function below is evalued with that b.

enter image description here

And in the result below with replacment the result is 4 instead of 2 as what I want.

enter image description here

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    $\begingroup$ You are trying to subvert a pretty fundamental part of the language. Try to describe the problem you are trying to solve, rather than the solution you are attempting to implement. I suspect there might be another way to achieve your original goal. $\endgroup$
    – MarcoB
    Apr 9, 2022 at 14:25
  • $\begingroup$ One workaround would be to use Clear[b] just before you call myfunction. Or to add Clear[b] into the function: myfunction[a_] := Module[{}, Clear[b]; 1 + (2 a - 1) b]. $\endgroup$
    – bill s
    Apr 9, 2022 at 15:26
  • $\begingroup$ @MarcoB I want to have a function that retun an expression with variables that I can replace later to get the value if needed. If I make b local then I cannot return b but something like $1000 and it's hard to replace for value later. ut $\endgroup$
    – hana
    Apr 9, 2022 at 16:24
  • $\begingroup$ @bills I did that too but I wondered if there is a better way as I used too many variables so I'm kind of lacking names. $\endgroup$
    – hana
    Apr 9, 2022 at 16:25

3 Answers 3

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Why not define your function with two input variables?

myfunction[a_, b_] := Module[{}, 1 + (2 a - 1) b ]

That way you can assignments like, enter image description here

Although it's a bit more complex if you want a two-variable function to assign one variable with the second deferred you could also use the CurryApplied function. This is a common feature in functional programming languages like Haskell. So you can define:

myFunctionB = CurryApplied[myFunction, 2][x];

which will assign x to the variable a in myFunction. And then at a later time, you can assign the variable b to myFunctionB:

myFunctionB[y]

which will return

1 + (-1 + 2 x) y
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If it is acceptable for you to return b in the function as a string instead of as a variable you could do the following:

f[a_]:=Module[{},  1 + (2 a - 1) "b" ]

Then you could just replace "b" instead of b:

f[1]/."b"->2
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Here's something you could do:

myfunction[a_] := Block[{b},  HoldForm[Evaluate[1 + (2 a - 1) b]]]

b = 2;
ReleaseHold[myfunction[1] /. Unevaluated[b -> 1]]

I don't recommend this style of programming, though. It's easy to mess up the holding/releasing of evaluations and get a wrong result. If you need variables that are supposed to not have values assigned to them, I recommend using Formal symbols instead, such as:

http://reference.wolfram.com/language/ref/character/FormalB.html

Edit

Another strategy for the last line would be:

Block[{b = 1}, ReleaseHold[myfunction[1]]]
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