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Tej Chajed
stdpp
Commits
4f546926
Commit
4f546926
authored
Feb 07, 2018
by
Robbert Krebbers
Browse files
A simple type class based canceler for natural numbers.
parent
392d08c6
Changes
2
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4f546926
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@@ 40,4 +40,5 @@ theories/functions.v
theories/hlist.v
theories/sorting.v
theories/infinite.v
theories/nat_cancel.v
theories/nat_cancel.v
0 → 100644
View file @
4f546926
From
stdpp
Require
Import
numbers
.
(* The class [NatCancel m n m' n'] is a simple canceler for natural numbers
implemented using type classes.
Input: [m], [n]; output: [m'], [n'].
It turns an equality [n = m] into an equality [n' = m'] by canceling out terms
that appear on both sides of the equality. We provide instances to handle the
following connectives over natural numbers:
n := 0  t  n + m  S m
Here, [t] represents arbitrary terms that do not fit the grammar. The instances
the class perform a depthfirst traversal (from left to right) through [n] and
try to cancel each leaf in [m]. This implies that the shape of the original
expressions [n] and [m] are preserved as much as possible. For example,
canceling:
S (S m2) + (k1 + (S k2 + k3)) + n1 = 2 + S ((n1 + S n2) + S (S m1 + m2))
Results in:
k1 + (k2 + k3) = S (n2 + S (S m1))
The instances are setup up so that canceling is performed in two stages.
 In the first stage, using the class [NatCancel], it traverses [m] w.r.t. [S]
and [+].
 In the second stage, for each leaf (i.e. a constant or arbitrary term)
obtained by the traversal in stage 1, it uses the class [NatCancelLeaf] to
cancel the leaf in [n].
Be warned: Since the canceler is implemented using type classes it should only
be used it either of the inputs is relatively small. For bigger inputs, an
approach based on reflection would be better, but for small inputs, the overhead
of reification will probably not be worth it. *)
Class
NatCancel
(
m
n
m'
n'
:
nat
)
:
=
nat_cancel
:
m'
+
n
=
m
+
n'
.
Hint
Mode
NatCancel
!
!


:
typeclass_instances
.
Class
NatCancelLeaf
(
m
n
m'
n'
:
nat
)
:
=
nat_cancel_leaf
:
NatCancel
m
n
m'
n'
.
Hint
Mode
NatCancelLeaf
!
!


:
typeclass_instances
.
Global
Existing
Instance
nat_cancel_leaf

100
.
Class
MakeNatS
(
n1
n2
m
:
nat
)
:
=
make_nat_S
:
m
=
n1
+
n2
.
Global
Instance
make_nat_S_0_l
n
:
MakeNatS
0
n
n
.
Proof
.
done
.
Qed
.
Global
Instance
make_nat_S_1
n
:
MakeNatS
1
n
(
S
n
).
Proof
.
done
.
Qed
.
Class
MakeNatPlus
(
n1
n2
m
:
nat
)
:
=
make_nat_plus
:
m
=
n1
+
n2
.
Global
Instance
make_nat_plus_0_l
n
:
MakeNatPlus
0
n
n
.
Proof
.
done
.
Qed
.
Global
Instance
make_nat_plus_0_r
n
:
MakeNatPlus
n
0
n
.
Proof
.
unfold
MakeNatPlus
.
by
rewrite
Nat
.
add_0_r
.
Qed
.
Global
Instance
make_nat_plus_default
n1
n2
:
MakeNatPlus
n1
n2
(
n1
+
n2
)

100
.
Proof
.
done
.
Qed
.
Global
Instance
nat_cancel_leaf_here
m
:
NatCancelLeaf
m
m
0
0

0
.
Proof
.
by
unfold
NatCancelLeaf
,
NatCancel
.
Qed
.
Global
Instance
nat_cancel_leaf_else
m
n
:
NatCancelLeaf
m
n
m
n

100
.
Proof
.
by
unfold
NatCancelLeaf
,
NatCancel
.
Qed
.
Global
Instance
nat_cancel_leaf_plus
m
m'
m''
n1
n2
n1'
n2'
n1'n2'
:
NatCancelLeaf
m
n1
m'
n1'
→
NatCancelLeaf
m'
n2
m''
n2'
→
MakeNatPlus
n1'
n2'
n1'n2'
→
NatCancelLeaf
m
(
n1
+
n2
)
m''
n1'n2'

2
.
Proof
.
unfold
NatCancelLeaf
,
NatCancel
,
MakeNatPlus
.
omega
.
Qed
.
Global
Instance
nat_cancel_leaf_S_here
m
n
m'
n'
:
NatCancelLeaf
m
n
m'
n'
→
NatCancelLeaf
(
S
m
)
(
S
n
)
m'
n'

3
.
Proof
.
unfold
NatCancelLeaf
,
NatCancel
.
omega
.
Qed
.
Global
Instance
nat_cancel_leaf_S_else
m
n
m'
n'
:
NatCancelLeaf
m
n
m'
n'
→
NatCancelLeaf
m
(
S
n
)
m'
(
S
n'
)

4
.
Proof
.
unfold
NatCancelLeaf
,
NatCancel
.
omega
.
Qed
.
(* The instance [nat_cancel_S_both] is redundant, but may reduce proof search
quite a bit, e.g. when canceling constants in constants. *)
Global
Instance
nat_cancel_S_both
m
n
m'
n'
:
NatCancel
m
n
m'
n'
→
NatCancel
(
S
m
)
(
S
n
)
m'
n'

1
.
Proof
.
unfold
NatCancel
.
omega
.
Qed
.
Global
Instance
nat_cancel_plus
m1
m2
m1'
m2'
m1'm2'
n
n'
n''
:
NatCancel
m1
n
m1'
n'
→
NatCancel
m2
n'
m2'
n''
→
MakeNatPlus
m1'
m2'
m1'm2'
→
NatCancel
(
m1
+
m2
)
n
m1'm2'
n''

2
.
Proof
.
unfold
NatCancel
,
MakeNatPlus
.
omega
.
Qed
.
Global
Instance
nat_cancel_S
m
m'
m''
Sm'
n
n'
n''
:
NatCancel
m
n
m'
n'
→
NatCancelLeaf
1
n'
m''
n''
→
MakeNatS
m''
m'
Sm'
→
NatCancel
(
S
m
)
n
Sm'
n''

3
.
Proof
.
unfold
NatCancelLeaf
,
NatCancel
,
MakeNatS
.
omega
.
Qed
.
\ No newline at end of file
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