How to clear all definitions matching f[1, ...] but not f[2, ...]?

Question

I would like to clear all definitions of the form f[1,...] in such a way that f[2,...] remain unaffected.

The two methods I know to clear variables don't seem to apply to this situation:

The second option works for particular values of f[1,...] but doesn't work with pattern matching.

$\begingroup$ Related, perhaps duplicate: (7972), (19536) $\endgroup$
– Mr.Wizard
Commented Jun 23, 2020 at 19:18 — Mr.Wizard, Commented Jun 23, 2020 at 19:18

Simon Woods · Accepted Answer · 2020-05-16 19:36:57Z

16

You can modify DownValues[f] directly. For example

DownValues[f] = 
 DeleteCases[DownValues[f], HoldPattern[_[f[1, ___]]] :> _]

answered May 16, 2020 at 19:36

Simon Woods

85.4k8 gold badges180 silver badges326 bronze badges

$\begingroup$ This works, but I'm a bit confused about the syntax _[...]. Could you explain why the f[1,___] needs brackets around it in this context? $\endgroup$
– WillG
Commented May 18, 2020 at 3:59
$\begingroup$ @WillG we need to match the HoldPattern as it appears in the DownValues, but without putting it explicitly in the pattern provided to DeleteCase. So we use a Blank. In other words, _[...] matches HoldPattern[...] but doesn't act as a held pattern. It looks confusing because then we have to wrap the pattern in HoldPattern to avoid evaluating the pattern. $\endgroup$
– Simon Woods
Commented May 18, 2020 at 19:43
$\begingroup$ Ok I think I see. So the HoldPattern that appears explicitly in HoldPattern[_[f[1,___]]] is not what gets matched against the HoldPattern stored in DownValues? $\endgroup$
– WillG
Commented May 18, 2020 at 21:42
$\begingroup$ @WillG exactly. $\endgroup$
– Simon Woods
Commented May 19, 2020 at 18:08

Add a comment |

1 Answer 1