BlankSequence with Plus

Question

When I execute f[g[a, b]] /. f[g[l__]] :> f[l] I get f[a, b] as expected.

When I execute f[a + b] /. f[Plus[l__]] :> f[l] I get f[a + b]. (I expect f[a, b] again)

Why? (Wolfram Mathematica 12.2.0)

Interesting: f[Plus[a, b]] /. f[g_[l1__]] :> f[l1] gives: f[a, b] — Daniel Huber, Mar 08 '21 at 12:50

score 11 · Answer 1 · answered Mar 08 '21 at 13:02

11

This happens because Plus[l__] evaluates to l__ before the pattern is even replaced. Try instead:

f[a + b] /. f[HoldPattern@Plus[l__]] :> f[l]

answered Mar 08 '21 at 13:02

1

you can also do f[a + b] /. f[l_Plus]] :> f[l] – Филя Усков Mar 08 '21 at 13:16
2

That still evaluates to f[a + b], right? – Fidel I. Schaposnik Mar 08 '21 at 13:20

1 Answers1