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It is customary to use the following idiom to partially evaluate something inside a held expression:

f[x_] := 2 x
Block[{f}, Hold[f[1 + 1]] /. x_f :> With[{r = x}, r /; True]]

Out[]= Hold[f[2]]

This works also with builtin symbols, like

Block[{Abs}, Hold[Abs[1 + 1]] /. x_Abs :> With[{r = x}, r /; True]]

Out[]= Hold[Abs[2]]

However, I cannot understand the behaviour of the following

Block[{Plus}, Hold[1 + 2*2] /. x_Plus :> With[{r = x}, r /; True]]

which returns Hold[5] instead of Hold[1+4]. Can someone explain why?

It is customary to use the following idiom to partially evaluate something inside a held expression:

f[x_] := 2 x
Block[{f}, Hold[f[1 + 1]] /. x_f :> With[{r = x}, r /; True]]

Out[]= Hold[f[2]]

This works also with built-in symbols, like

Block[{Abs}, Hold[Abs[1 + 1]] /. x_Abs :> With[{r = x}, r /; True]]

Out[]= Hold[Abs[2]]

However, I cannot understand the behaviour of the following

Block[{Plus}, Hold[1 + 2*2] /. x_Plus :> With[{r = x}, r /; True]]

which returns Hold[5] instead of Hold[1+4]. Can someone explain why?

It is customary to use the following idiom to partially evaluate something inside a held expression:

f[x_] := 2 x
Block[{f}, Hold[f[1 + 1]] /. x_f :> With[{r = x}, r /; True]]

Out[]= Hold[f[2]]

This works also with builtin symbols, like

Block[{Abs}, Hold[Abs[1 + 1]] /. x_Abs :> With[{r = x}, r /; True]]

Out[]= Hold[Abs[2]]

However, I cannot understand the behaviour of the following

Block[{Plus}, Hold[1 + 2*2] /. x_Plus :> With[{r = x}, r /; True]]

which returns Hold[7] instead of Hold[1+6]. Can someone explain why?

It is customary to use the following idiom to partially evaluate something inside a held expression:

f[x_] := 2 x
Block[{f}, Hold[f[1 + 1]] /. x_f :> With[{r = x}, r /; True]]

Out[]= Hold[f[2]]

This works also with builtin symbols, like

Block[{Abs}, Hold[Abs[1 + 1]] /. x_Abs :> With[{r = x}, r /; True]]

Out[]= Hold[Abs[2]]

However, I cannot understand the behaviour of the following

Block[{Plus}, Hold[1 + 2*2] /. x_Plus :> With[{r = x}, r /; True]]

which returns Hold[5] instead of Hold[1+4]. Can someone explain why?