Substitute Values of Variables in Functions That Hold Their Arguments

Define a held expression that refers to the variable x:

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Because Hold has the attribute HoldAll, x is not evaluated when the held expression is defined, so the body of the expression refers to the global variable x. The value of x is obtained when the Hold is released:

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To effectively force x to evaluate when the held expression is defined, use With:

Now changes in the value of x have no effect on the value returned when the Hold is released:

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Notes

This technique applies to any function with HoldAll, HoldFirst, HoldRest or HoldAllComplete attributes, for example, Function itself:
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Evaluate can be used to resolve a variable to its value in a held argument, but only when it occurs at the top level of the argument. Compare: