Assume in lean 4 - lean

Question

A lot of proofs in Lean 3 were structured via the assume syntax e.g.
theorem WetTheorem :
forall Rain Hydrant Wet: Prop,
(Rain ∨ Hydrant) → -- raining or hydrant on;
(Rain → Wet) → -- if raining then wet;
(Hydrant → Wet) → -- if hydrant on then wet;
Wet -- then wet
:=
begin
-- setup
assume Rain Hydrant Wet,
assume RainingOrHydrantRunning: (Rain ∨ Hydrant),
assume RainMakesWet: (Rain → Wet),
assume HydrantMakesWet: (Hydrant → Wet),
-- the core of the proof
cases RainingOrHydrantRunning with raining running,
show Wet, from RainMakesWet raining,
show Wet, from HydrantMakesWet running,
end
I was unable find a similar reference section for tactics in lean 4 like in 3, and it seems assume has been dropped. What would be the general method of converting a lean 3 proof with assume into a lean 4 without it?

Answer 1

assume in tactic mode can be replaced by intro with a type ascription in lean 4, so your entire code can be written as follows
theorem WetTheorem :
forall Rain Hydrant Wet: Prop,
(Rain ∨ Hydrant) → -- raining or hydrant on;
(Rain → Wet) → -- if raining then wet;
(Hydrant → Wet) → -- if hydrant on then wet;
Wet -- then wet
:=
by
-- setup
intro Rain Hydrant Wet
intro (RainingOrHydrantRunning : (Rain ∨ Hydrant))
intro (RainMakesWet: (Rain → Wet))
intro (HydrantMakesWet: (Hydrant → Wet))
-- the core of the proof
cases RainingOrHydrantRunning
case inl raining =>
exact RainMakesWet raining
case inr running =>
exact HydrantMakesWet running