# Library Coq.Structures.OrderedTypeAlt

Require Import OrderedType.

# An alternative (but equivalent) presentation for an Ordered Type

inferface.
NB: comparison, defined in Datatypes.v is Eq|Lt|Gt whereas compare, defined in OrderedType.v is EQ _ | LT _ | GT _
Module Type OrderedTypeAlt.
Parameter t : Type.
Parameter compare : t -> t -> comparison.
Infix "?=" := compare (at level 70, no associativity).
Parameter compare_sym :
forall x y, (y?=x) = CompOpp (x?=y).
Parameter compare_trans :
forall c x y z, (x?=y) = c -> (y?=z) = c -> (x?=z) = c.
End OrderedTypeAlt.
From this new presentation to the original one.
Module OrderedType_from_Alt (O:OrderedTypeAlt) <: OrderedType.
Import O.
Definition t := t.
Definition eq x y := (x?=y) = Eq.
Definition lt x y := (x?=y) = Lt.
Lemma eq_refl : forall x, eq x x.
Lemma eq_sym : forall x y, eq x y -> eq y x.
Definition eq_trans := (compare_trans Eq).
Definition lt_trans := (compare_trans Lt).
Lemma lt_not_eq : forall x y, lt x y -> ~eq x y.
Definition compare : forall x y, Compare lt eq x y.
Definition eq_dec : forall x y, { eq x y } + { ~ eq x y }.
End OrderedType_from_Alt.
From the original presentation to this alternative one.
Module OrderedType_to_Alt (O:OrderedType) <: OrderedTypeAlt.
Import O.
Module MO:=OrderedTypeFacts(O).
Import MO.
Definition t := t.
Definition compare x y := match compare x y with
| LT _ => Lt
| EQ _ => Eq
| GT _ => Gt
end.
Infix "?=" := compare (at level 70, no associativity).
Lemma compare_sym :
forall x y, (y?=x) = CompOpp (x?=y).
Lemma compare_trans :
forall c x y z, (x?=y) = c -> (y?=z) = c -> (x?=z) = c.
End OrderedType_to_Alt.