When evaluatingf[x_:x0] := x, x0 will be evaluated, regardless of the HoldAll attribute of SetDelayed.
Why the second argument of Optional is evaluated in a non-standard manner?
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$\begingroup$ Can you show us how the evaluation is non-standard? $\endgroup$– berniethejetFeb 29, 2020 at 15:09
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2$\begingroup$ Related: mathematica.stackexchange.com/a/95583/1871 $\endgroup$– xzczdFeb 29, 2020 at 15:46
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1 Answer
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From the "Details" of the docs for HoldPattern
The left-hand sides of rules are usually evaluated, as are parts of the left-hand sides of assignments. You can use
HoldPatternto stop any part from being evaluated.
Application:
ClearAll[f];
f[HoldPattern[x_: x0]] := x;
x0 = 7;
f[5]
f[]
(*
5
7
*)
Block[{x0 = 3},
f[]
]
(* 3 *)
answered Feb 29, 2020 at 15:10