## Stream: lean4

### Topic: Tactics

#### Calvin Lee (Jan 28 2021 at 23:15):

I'm pretty new to tactics, so please excuse my ignorance.
What's the difference between rw and erw? Also does simp not work on lean4? I'm getting a sorry when I try to use it.

#### Mario Carneiro (Jan 28 2021 at 23:18):

simp doesn't exist in lean 4 yet

#### Mario Carneiro (Jan 28 2021 at 23:19):

the docstring for erw says

/--
A variant of rewrite that uses the unifier more aggressively, unfolding semireducible definitions.
-/


#### Calvin Lee (Jan 28 2021 at 23:21):

Ok, so is there no way to, say, try a tactic in several places until it succeeds then?
e.g. if I wanted to apply rw [Nat.mulComm] but on the second or third term where it would be valid?

#### Mario Carneiro (Jan 28 2021 at 23:22):

this is not really a lean 4 question, but you can use rw [Nat.mulComm a] or rw [Nat.mulComm a b] to limit the matches to the one you want

#### Daniel Selsam (Jan 28 2021 at 23:23):

Mario Carneiro said:

simp doesn't exist in lean 4 yet

FYI it is still missing some features but much of the functionality is already there, e.g. https://github.com/leanprover/lean4/blob/master/tests/lean/run/simp4.lean

#### Calvin Lee (Jan 28 2021 at 23:26):

Ok, I'll try to post in #new members about tactic proofs. I wasn't sure if I would run into lean4 specific behavior that would send me back here :smile:

#### Kyle Miller (Jan 28 2021 at 23:30):

A new Lean 4 thing you can do is rw [Nat.mulComm (m := a)] rather than rw [Nat.mulComm _ a], supposing you knew m was the name of the second argument. This is more useful for lemmas with multiple arguments, especially implicit ones.

Last updated: May 18 2021 at 22:15 UTC