LHS & RHS

LHS means “left-hand side”. It returns the term on the left hand side of an equality or inequality.

Ltac LHS :=
match goal with
| |-(?a = _) => constr: a
| |-(_ ?a _) => constr: a
end.

For example you can use this to write a tactic that proves the cofixpoint equations of streams:

Ltac decomp_stream := intros; let L := LHS in rewrite (Streams.unfold_Stream L); reflexivity.

RHS means “right-hand side”. It returns the term on the right hand side of an equality or inequality.

Ltac RHS :=
match goal with
| |-(_ = ?a) => constr: a
| |-(_ _ ?a) => constr: a
end.

We can make special tactics for replace LHS and replace RHS that give similar functionality to step.

Tactic Notation "replace" "LHS" "with" constr (a) "by" tactic (t) :=
match goal with
| |-(?r ?b ?c) =>
  let Z := fresh "Z" in
  (change (let Z:=b in r Z c);intro Z;setoid_replace Z with a;
   [unfold Z; clear Z|unfold Z; clear Z; solve [ t ]])
end.
Tactic Notation "replace" "LHS" "with" constr (a) :=
match goal with
| |-(?r ?b ?c) =>
  let Z := fresh "Z" in
  (change (let Z:=b in r Z c);intro Z;setoid_replace Z with a;
   unfold Z; clear Z)
end.
Tactic Notation "replace" "RHS" "with" constr (a) "by" tactic (t) :=
match goal with
| |-(?r ?b ?c) =>
  let Z := fresh "Z" in
  (change (let Z:=c in r b Z);intro Z;setoid_replace Z with a;
   [unfold Z; clear Z|unfold Z; clear Z; solve [ t ]])
end.
Tactic Notation "replace" "RHS" "with" constr (a) :=
match goal with
| |-(?r ?b ?c) =>
  let Z := fresh "Z" in
  (change (let Z:=c in r b Z);intro Z;setoid_replace Z with a;
   unfold Z; clear Z)
end.