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Fengmin Zhu
Tutorial POPL20
Commits
db309714
Commit
db309714
authored
Jan 20, 2020
by
Robbert Krebbers
Browse files
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parent
3ad58265
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theories/polymorphism.v
View file @
db309714
From
tutorial_popl20
Require
Export
language
.
(** * Polymorphism and existential types in HeapLang *)
(** Since HeapLang is an untyped language, it does not have constructs for
polymorphic functions (i.e. typelevel lambdas and application), and for
existential types (i.e. pack and unpack). *)
(** In order to define a type system for HeapLang (in the file [typed.v]) we
need to extend HeapLang with constructs for polymorphic functions (i.e.
typelevel lambdas and application), and for existential types (i.e. pack and
unpack). Since HeapLang is an untyped language, it does natively have these
constructs. *)
(** We retrofit typelevel lambdas and application on HeapLang by defining them
as mere thunks, and ordinary application, respectively. This ensures that
polymorphic programs satisfy a **value
condi
tion**, which is needed to obtain
polymorphic programs satisfy a **value
restric
tion**, which is needed to obtain
type safety in the presence of mutable state. *)
Notation
"Λ: e"
:
=
(
λ
:
<>,
e
)%
E
(
at
level
200
,
only
parsing
)
:
expr_scope
.
Notation
"Λ: e"
:
=
(
λ
:
<>,
e
)%
V
(
at
level
200
,
only
parsing
)
:
val_scope
.
...
...
theories/typed.v
View file @
db309714
From
tutorial_popl20
Require
Export
types
.
From
tutorial_popl20
Require
Export
types
polymorphism
.
(** * Syntactic typing for HeapLang *)
(** In this file, we define a syntactic type system for HeapLang. We do this
in the conventional way by defining the typing judgment [Γ ⊢ₜ e : τ] using an
inductive relation. *)
(** * Operator typing *)
(** In order to define the typing judgment, we first need some helpers for
operator typing. *)
(** The first helper we define is [ty_unboxed τ], which expresses that values
of [τ] are unboxed, i.e. they fit into one machine word. This helper is needed
to state the typing rules for equality and compareandexchange ([CmpXchg]),
which can only be used on unboxed values. *)
Inductive
ty_unboxed
:
ty
→
Prop
:
=

TUnit_unboxed
:
ty_unboxed
TUnit

TBool_unboxed
:
ty_unboxed
TBool

TInt_unboxed
:
ty_unboxed
TInt

TRef_unboxed
τ
:
ty_unboxed
(
TRef
τ
).
(** In order to let Coq automatically prove that types are unboxed, we turn
[ty_unboxed] into a type class, and turn the constructors into type class
instances. This is done using the following commands. *)
Existing
Class
ty_unboxed
.
Existing
Instances
TUnit_unboxed
TBool_unboxed
TInt_unboxed
TRef_unboxed
.
(** We can now use Coq's type class inference mechanism to automatically
establish that given types are unboxed. This is done by invoking the [apply _]
tactic. *)
Lemma
TRef_TRef_TInt_unboxed
:
ty_unboxed
(
TRef
(
TRef
TInt
)).
Proof
.
apply
_
.
Qed
.
(** The true power of turning [ty_unboxed] into a type class is that whenever a
lemma or definition has a [ty_unboxed] argument, type class inference is called
automatically. *)
(** The relation [ty_un_op op τ σ] expresses that a unary operator [op] with an
argument of type [τ] has result type [σ]. We also turn [ty_un_op] into a type
class. *)
Inductive
ty_un_op
:
un_op
→
ty
→
ty
→
Prop
:
=

Ty_un_op_int
op
:
ty_un_op
op
TInt
TInt

Ty_un_op_bool
:
ty_un_op
NegOp
TBool
TBool
.
Existing
Class
ty_un_op
.
Existing
Instances
Ty_un_op_int
Ty_un_op_bool
.
(** The relation [ty_bin_op op τ1 τ2 σ] expresses that a binary operator [op]
with arguments of type [τ1] and [τ2] has result type [σ]. In order to avoid
an abundance of rules, we factorize the operators into 4 categories: equality,
arithmetic, comparison, and Boolean operators. For the last 3 categories, we make
use of [TCElemOf x xs], where [x : A] and [xs : list A], which is a type class
version of [x ∈ xs]. *)
Inductive
ty_bin_op
:
bin_op
→
ty
→
ty
→
ty
→
Prop
:
=

Ty_bin_op_eq
τ
:
ty_unboxed
τ
→
ty_bin_op
EqOp
τ
τ
TBool
...
...
@@ 28,6 +64,78 @@ Inductive ty_bin_op : bin_op → ty → ty → ty → Prop :=
Existing
Class
ty_bin_op
.
Existing
Instances
Ty_bin_op_eq
Ty_bin_op_arith
Ty_bin_op_compare
Ty_bin_op_bool
.
(** * The typing judgment *)
(** With the above helpers at hand, we can define the syntactic typing judgment
[Γ ⊢ₜ e : τ]. While most of the typing rules are standard, the definition
involves a number of interesting aspects.
 Since termlevel variables in HeapLang are modeled using strings, we represent
typing contexts [Γ : gmap string ty] as mappings from strings to types. Here,
[gmap] is the type of generic maps from the std++ library.
 In addition to named binders, HeapLang also features the anonymous binder
[<>]. This allows one to define [λ: x, e] as [rec: <> x := e]. Binders in
HeapLang are of type [binder], whose definition is as follows:
<<
Inductive binder := BAnon : binder  BNamed : string → binder.
>>
As a result, in the typing rules of all constructs that involve binders (i.e.,
the typing rules [Rec_typed] and [Unpack_typed]) we have to consider two
cases [BAnon] and [BNamed]. To factorize these typing rules, we make use of
[binder_insert], which lifts the insert operator [<[_:=_> _] on [gmap] to
binders.
 The type of values [val] and expressions [expr] of HeapLang are defined in
a mutually inductive fashion:
<<
Inductive expr :=
(* Values *)
 Val (v : val)
(* Base lambda calculus *)
 Var (x : string)
 Rec (f x : binder) (e : expr)
 App (e1 e2 : expr)
(* Products *)
 Pair (e1 e2 : expr)
 Fst (e : expr)
 Snd (e : expr)
(* Sums *)
 InjL (e : expr)
 InjR (e : expr)
 Case (e0 : expr) (e1 : expr) (e2 : expr)
(* Etc. *)
with val :=
 LitV (l : base_lit)
 RecV (f x : binder) (e : expr)
 PairV (v1 v2 : val)
 InjLV (v : val)
 InjRV (v : val).
>>
For technical reasons, the only terms that are considered values are those
that begin with the [Val] expression former. This means that, for example,
[Pair (Val v1) (Val v2)] is not a valueit reduces to [Val (PairV v1 v2)],
which is. This leads to some administrative redexes, and to a distinction
between "value pairs", "value sums", "value closures" and their "expression"
counterparts.
However, this also makes values syntactically uniform, which we exploit in the
definition of substitution ([subst]), which just skips over [Val] terms,
because values should be closed, and hence not affected by substitution. As a
consequence, we can entirely avoid talking about "closed terms" in the
definition of HeapLang.
As a result of the mutual inductive definition, and the distinction between
"value pairs", "value sums", "value closures" and their "expression"
counterparts, we need to define the typing judgment in a mutual inductive
fashion too. Hence, apart from the judgment [Γ ⊢ₜ e : τ], we have the judgment
[⊢ᵥ v : τ]. Note that since values are supposed to be closed, the latter
judgment does not have a context [Γ].
*)
(** We use [Reserved Notation] so we can use the notation already in the
definition of the typing judgment. *)
Reserved
Notation
"Γ ⊢ₜ e : τ"
(
at
level
74
,
e
,
τ
at
next
level
).
Reserved
Notation
"⊢ᵥ v : τ"
(
at
level
20
,
v
,
τ
at
next
level
).
...
...
@@ 141,10 +249,25 @@ with val_typed : val → ty → Prop :=
where
"Γ ⊢ₜ e : τ"
:
=
(
typed
Γ
e
τ
)
and
"⊢ᵥ v : τ"
:
=
(
val_typed
v
τ
).
(** * Exercise (suger_typed, easy) *)
(** To make it possible to write programs in a compact way, HeapLang features
the following syntactic sugar:
<<
(λ: x, e) := (rec: <> x := e)
(let: x := e1 in e2) := (λ x, e2) e1
(e1 ;; e2) := (let: <> := e1 in e2)
Skip := (λ: <>, #()) #()
>>
Prove the following derived typing rules for the syntactic sugar. Note that
due to the distinction between expressions and values, you have to prove some
of them for both the expression construct and their value counterpart. *)
Lemma
Lam_typed
Γ
x
e
τ
1
τ
2
:
binder_insert
x
τ
1
Γ
⊢
ₜ
e
:
τ
2
→
Γ
⊢
ₜ
(
λ
:
x
,
e
)
:
TArr
τ
1
τ
2
.
Proof
.
(* REMOVE *)
Proof
.
intros
He
.
apply
Rec_typed
.
simpl
.
...
...
@@ 154,7 +277,7 @@ Qed.
Lemma
LamV_typed
x
e
τ
1
τ
2
:
binder_insert
x
τ
1
∅
⊢
ₜ
e
:
τ
2
→
⊢
ᵥ
(
λ
:
x
,
e
)
:
TArr
τ
1
τ
2
.
Proof
.
(* REMOVE *)
Proof
.
intros
He
.
apply
RecV_typed
.
simpl
.
...
...
@@ 165,7 +288,7 @@ Lemma Let_typed Γ x e1 e2 τ1 τ2 :
Γ
⊢
ₜ
e1
:
τ
1
→
binder_insert
x
τ
1
Γ
⊢
ₜ
e2
:
τ
2
→
Γ
⊢
ₜ
(
let
:
x
:
=
e1
in
e2
)
:
τ
2
.
Proof
.
(* REMOVE *)
Proof
.
intros
He1
He2
.
apply
App_typed
with
τ
1
.

by
apply
Lam_typed
.
...
...
@@ 176,21 +299,25 @@ Lemma Seq_typed Γ e1 e2 τ1 τ2 :
Γ
⊢
ₜ
e1
:
τ
1
→
Γ
⊢
ₜ
e2
:
τ
2
→
Γ
⊢
ₜ
(
e1
;;
e2
)
:
τ
2
.
Proof
.
(* REMOVE *)
Proof
.
intros
He1
He2
.
by
apply
Let_typed
with
τ
1
.
Qed
.
Lemma
Skip_typed
Γ
:
Γ
⊢
ₜ
Skip
:
().
Proof
.
(* REMOVE *)
Proof
.
apply
App_typed
with
()%
ty
.

apply
Val_typed
,
RecV_typed
.
apply
Val_typed
,
UnitV_typed
.

apply
Val_typed
,
UnitV_typed
.
Qed
.
(** * Typing of concrete programs *)
(** ** Exercise (swap_typed, easy) *)
(** Prove that the nonpolymorphic swap function [swap] can be given the type
[ref τ → ref τ → ()] for any [τ]. *)
Lemma
swap_typed
τ
:
⊢
ᵥ
swap
:
(
ref
τ
→
ref
τ
→
()).
Proof
.
(* REMOVE *)
Proof
.
rewrite
/
swap
.
apply
LamV_typed
.
apply
Lam_typed
.
...
...
@@ 205,8 +332,11 @@ Proof.

by
apply
Var_typed
.
Qed
.
(** ** Exercise (swap_poly_typed, easy) *)
(** Prove that [swap_poly] can be typed using the polymorphic type
[∀ X, ref X → ref X → ())], i.e. [∀: ref #0 → ref #0 → ())] in De Bruijn style. *)
Lemma
swap_poly_typed
:
⊢
ᵥ
swap_poly
:
(
∀:
ref
#
0
→
ref
#
0
→
()).
Proof
.
(* REMOVE *)
Proof
.
rewrite
/
swap_poly
.
apply
TLamV_typed
.
do
2
apply
Lam_typed
.
...
...
@@ 221,8 +351,11 @@ Proof.

by
apply
Var_typed
.
Qed
.
Lemma
unsafe_pure_not_typed
Γ
τ
:
¬
(
Γ
⊢
ₜ
unsafe_pure
:
τ
).
Proof
.
(** ** Exercise (not_typed, easy) *)
(** Prove that the programs [unsafe_pure] and [unsafe_ref] from [language.v]
cannot be typed using the syntactic type system. *)
Lemma
unsafe_pure_not_typed
Γ
τ
:
¬
(
Γ
⊢
ᵥ
unsafe_pure
:
τ
).
(* REMOVE *)
Proof
.
intros
Htyped
.
repeat
match
goal
with
...
...
@@ 231,8 +364,8 @@ Proof.
end
.
Qed
.
Lemma
unsafe_ref_not_typed
Γ
τ
:
¬
(
Γ
⊢
ₜ
unsafe_ref
:
τ
).
Proof
.
Lemma
unsafe_ref_not_typed
Γ
τ
:
¬
(
Γ
⊢
ᵥ
unsafe_ref
:
τ
).
(* REMOVE *)
Proof
.
intros
Htyped
.
repeat
match
goal
with
...
...
theories/types.v
View file @
db309714
From
tutorial_popl20
Require
Export
polymorphism
.
From
tutorial_popl20
Require
Export
language
.
(** * Syntactic types for HeapLang *)
(** The inductive type [ty] defines the syntactic types for HeapLang. We make
use of De Bruijn indices to support typelevel binding. *)
Inductive
ty
:
=

TVar
:
nat
→
ty

TUnit
:
ty
...
...
@@ 12,8 +15,14 @@ Inductive ty :=

TExist
:
ty
→
ty

TRef
:
ty
→
ty
.
Delimit
Scope
ty_scope
with
ty
.
(** To obtain nice notations when writing types for concrete programs, we define
some Coq notations for types. These notations are put in the notation scope
[ty_scope]. We use the [Bind Scope] command to instruct Coq to parse terms of
type [ty] using the notations in [type_scope]. When the type is not clear from
the context, one can write [τ%ty] to force Coq to parse [τ] using the notations
in [type_scope]. This is set up using the [Delimit Scope] command. *)
Bind
Scope
ty_scope
with
ty
.
Delimit
Scope
ty_scope
with
ty
.
Notation
"# x"
:
=
(
TVar
x
)
:
ty_scope
.
Notation
"()"
:
=
TUnit
:
ty_scope
.
Infix
"*"
:
=
TProd
:
ty_scope
.
...
...
@@ 23,7 +32,10 @@ Notation "∀: τ" := (TForall τ) (at level 100, τ at level 200) : ty_scope.
Notation
"∃: τ"
:
=
(
TExist
τ
)
(
at
level
100
,
τ
at
level
200
)
:
ty_scope
.
Notation
"'ref' τ"
:
=
(
TRef
τ
)
(
at
level
10
,
τ
at
next
level
,
right
associativity
)
:
ty_scope
.
(** De Bruijn substitution *)
(** * Typelevel substitution *)
(** Below we define the function [ty_subst x σ τ], which replaces the De Bruijn
index [x] in the type [τ] by the type [σ]. The definition is standard, and makes
use of the lifting function [ty_lift]. *)
Fixpoint
ty_lift
(
n
:
nat
)
(
τ
:
ty
)
:
ty
:
=
match
τ
with

TVar
y
=>
TVar
(
if
decide
(
y
<
n
)
then
y
else
S
y
)%
nat
...
...
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