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Pierre Letouzey
coqlmfi
Commits
cfb708ae
Commit
cfb708ae
authored
Dec 02, 2020
by
Pierre Letouzey
Browse files
a variant of TD5 exo1 with even more dependent types
parent
f6f06d74
Changes
1
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solutions/td5b.v
0 → 100644
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cfb708ae
(
*
TD5
*
)
Require
Import
Arith
List
.
Import
ListNotations
.
Set
Implicit
Arguments
.
(
*
Exercise
2
of
TD4
,
this
time
with
dependent
types
expressing
that
trees
are
necessarily
perfect
+
strictly
sorted
by
depth
Thanks
to
Enrique
for
the
initial
idea
.
*
)
Module
BList_via_HollowIndexedList
.
Inductive
fulltree
(
A
:
Type
)
:
nat
>
Type
:=

FLeaf
:
A
>
fulltree
A
0

FNode
n
:
fulltree
A
n
>
fulltree
A
n
>
fulltree
A
(
S
n
).
Arguments
FLeaf
{
A
}
.
Arguments
FNode
{
A
}
{
n
}
.
(
*
Hilist
:
hollow
+
indexed
lists
.
Hollow
:
an
element
may
be
skipped
via
the
Hole
constructor
.
Indexed
:
if
present
,
the
n

th
slot
of
a
(
hilist
B
k
)
has
type
(
B
(
n
+
k
)).
Roughly
,
hilist
=
list
(
option
..)
where
the
types
of
elements
are
dependent
.
*
)
Inductive
hilist
(
B
:
nat
>
Type
)
:
nat
>
Type
:=

Nil
n
:
hilist
B
n

Hole
n
:
hilist
B
(
S
n
)
>
hilist
B
n

Cons
n
:
B
n
>
hilist
B
(
S
n
)
>
hilist
B
n
.
Arguments
Nil
{
B
}
{
n
}
.
Arguments
Hole
{
B
}
{
n
}
.
Arguments
Cons
{
B
}
{
n
}
.
Definition
innerblist
A
n
:=
hilist
(
fulltree
A
)
n
.
Definition
blist
A
:=
innerblist
A
0.
Definition
empty_blist
{
A
}
:
blist
A
:=
Nil
.
Definition
size1_blist
{
A
}
(
a
:
A
)
:
blist
A
:=
Cons
(
FLeaf
a
)
Nil
.
Definition
size2_blist
{
A
}
(
a
b
:
A
)
:
blist
A
:=
Hole
(
Cons
(
FNode
(
FLeaf
a
)
(
FLeaf
b
))
Nil
).
(
*
etc
.
The
size
of
the
trees
are
the
binary
decomposition
of
the
total
number
of
elements
in
the
blist
.
Eg
:
1
=
1
2
=
0
+
2
3
=
1
+
2
4
=
0
+
0
+
2
Here
,
zeros
in
the
decomposition
are
coded
by
constructor
Hole
.
*
)
(
*
Important
auxiliary
function
:
putting
a
tree
on
the
left
of
a
bl
.
*
)
(
*
invariant
:
depth
t
=
depth
of
leftmost
option
tree
in
the
bl
*
)
Fixpoint
constree
{
A
}
{
n
:
nat
}
(
bl
:
innerblist
A
n
)
:
fulltree
A
n
>
innerblist
A
n
:=
match
bl
with

Nil
=>
fun
t
=>
Cons
t
Nil

Hole
bl
'
=>
fun
t
=>
Cons
t
bl
'

Cons
t
'
bl
'
=>
fun
t
=>
Hole
(
constree
bl
'
(
FNode
t
t
'
))
end
.
Definition
cons
{
A
}
(
a
:
A
)
bl
:
blist
A
:=
constree
bl
(
FLeaf
a
).
Compute
cons
2
(
cons
3
(
size2_blist
5
7
)).
(
*
digression
:
from
regular
list
to
blist
and
back
*
)
Fixpoint
list_to_blist
{
A
}
(
l
:
list
A
)
:
blist
A
:=
match
l
with

[]
=>
empty_blist

x
::
l
=>
cons
x
(
list_to_blist
l
)
end
.
Compute
list_to_blist
[
1
;
2
;
3
;
4
;
5
;
6
;
7
].
Fixpoint
tree_to_list
{
A
}
{
n
}
(
t
:
fulltree
A
n
)
:
list
A
:=
match
t
with

FLeaf
a
=>
[
a
]

FNode
t
t
'
=>
tree_to_list
t
++
tree_to_list
t
'
end
.
Fixpoint
innerblist_to_list
{
A
}
{
n
}
(
bl
:
innerblist
A
n
)
:=
match
bl
with

Nil
=>
[]

Hole
bl
=>
innerblist_to_list
bl

Cons
t
bl
'
=>
tree_to_list
t
++
innerblist_to_list
bl
'
end
.
Definition
blist_to_list
{
A
}
(
bl
:
blist
A
)
:=
innerblist_to_list
bl
.
(
*
end
of
digression
*
)
Fixpoint
unconstree
{
A
}
{
n
}
(
t
:
fulltree
A
n
)
:
innerblist
A
n
>
A
*
blist
A
:=
match
t
with

FLeaf
a
=>
fun
bl
=>
(
a
,
bl
)

FNode
t
t
'
=>
fun
bl
=>
unconstree
t
(
Cons
t
'
bl
)
end
.
Fixpoint
inner_uncons
{
A
}
{
n
}
(
bl
:
innerblist
A
n
)
:
option
(
A
*
blist
A
)
:=
match
bl
with

Nil
=>
None

Hole
bl
=>
inner_uncons
bl

Cons
t
bl
=>
Some
(
unconstree
t
(
Hole
bl
))
end
.
Definition
uncons
{
A
}
(
bl
:
blist
A
)
:
option
(
A
*
blist
A
)
:=
inner_uncons
bl
.
(
*
invariant
:
n
<
size
t
(
which
is
2
^
depth
t
)
*
)
Fixpoint
nthtree
{
A
}
{
d
}
(
t
:
fulltree
A
d
)
n
:
option
A
:=
match
t
with

FLeaf
a
=>
Some
a
(
*
normally
here
n
must
be
0
*
)

FNode
t1
t2
=>
let
size_t1
:=
2
^
(
pred
d
)
in
if
n
<?
size_t1
then
nthtree
t1
n
else
nthtree
t2
(
n

size_t1
)
end
.
Fixpoint
inner_nth
{
A
}
{
d
}
(
bl
:
innerblist
A
d
)
n
pow2
:
option
A
:=
match
bl
with

Nil
=>
None

Hole
bl
'
=>
inner_nth
bl
'
n
(
2
*
pow2
)

Cons
t
bl
'
=>
if
n
<?
pow2
then
nthtree
t
n
else
inner_nth
bl
'
(
n

pow2
)
(
2
*
pow2
)
end
.
Definition
nth
{
A
}
(
bl
:
blist
A
)
n
:=
inner_nth
bl
n
1.
Compute
nth
(
list_to_blist
[
1
;
2
;
3
;
4
;
5
;
6
;
7
])
6.
End
BList_via_HollowIndexedList
.
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