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improved typing and trans closure in Lambda

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This commit is contained in:
wadler 2018-10-29 12:15:31 +00:00
parent 649d7dcb13
commit 9b0069d8d1
2 changed files with 44 additions and 45 deletions
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20
hs/agda-count.hs
View file

@ -21,21 +21,21 @@ ex1 = "xbyexxbyyexxxbyyyexxxx"
test1 :: Bool
test1 = strip (== 'b') (== 'e') ex1 == ["y","yy","yyy",""]
count :: (a -> Bool) -> (a -> Bool) -> [a] -> Int
count b e = sum . map length . strip b e
test2 :: Bool
test2 = count (== 'b') (== 'e') ex1 == 6
agda :: String -> Int
agda = count (prefix begin) (prefix end) . lines
where
begin = "\\begin{code}"
end = "\\end{code}"
test2 = (sum . map length . strip (== 'b') (== 'e')) ex1 == 6
nonblank :: String -> Bool
nonblank = not . all (== ' ')
count :: [[String]] -> Int
count = sum . map length . map (filter nonblank)
agda :: String -> Int
agda = count . strip begin end . lines
where
begin = prefix "\\begin{code}"
end = prefix "\\end{code}"
wc :: String -> Int
wc = length . lines

69
src/plfa/Lambda.lagda
View file

@ -700,29 +700,29 @@ and transitive. We could do so as follows.
\begin{code}
data _—↠′_ : Term → Term → Set where
step : ∀ {M N}
step′ : ∀ {M N}
→ M —→ N
-------
→ M —↠′ N
refl : ∀ {M}
refl′ : ∀ {M}
-------
→ M —↠′ M
trans : ∀ {L M N}
trans′ : ∀ {L M N}
→ L —↠′ M
→ M —↠′ N
-------
→ L —↠′ N
\end{code}
The three constructors specify, respectively, that `—↠` includes `—→`
The three constructors specify, respectively, that `—↠′` includes `—→`
and is reflexive and transitive. A good exercise is to show that
the two definitions are equivalent (indeed, isomoprhic).
the two definitions are equivalent (indeed, one embeds in the other).
#### Exercise `—↠≃—↠′`
#### Exercise `—↠≲—↠′`
Show that the two notions of reflexive and transitive closure
above are isomorphic.
Show that the first notion of reflexive and transitive closure
above embeds into the second. Why are they not isomorphic?
## Confluence
@ -1141,36 +1141,35 @@ evidence of _any_ proposition whatsoever, regardless of its truth.
Type derivations correspond to trees. In informal notation, here
is a type derivation for the Church numberal two:
∋s ∋z
-------------------- ⊢` -------------- ⊢`
∋s Γ₂ ⊢ ` "s" ⦂ `ℕ ⇒ `ℕ Γ₂ ⊢ ` "z" ⦂ `ℕ
-------------------- ⊢` ------------------------------------------ _·_
Γ₂ ⊢ ` "s" ⦂ `ℕ ⇒ `ℕ Γ₂ ⊢ ` "s" · ` "z" ⦂ `ℕ
------------------------------------------------ _·_
Γ₂ ⊢ ` "s" · (` "s" · ` "z") ⦂ `ℕ
---------------------------------------------- ⊢ƛ
Γ₁ ⊢ ƛ "z" ⇒ ` "s" · (` "s" · ` "z") ⦂ `ℕ ⇒ `ℕ
----------------------------------------------------------------- ⊢ƛ
∅ ⊢ ƛ "s" ⇒ ƛ "z" ⇒ ` "s" · (` "s" · ` "z") ⦂ (`ℕ ⇒ `ℕ) ⇒ `ℕ ⇒ `ℕ
∋s ∋z
------------------ ⊢` -------------- ⊢`
∋s Γ₂ ⊢ ` "s" ⦂ A ⇒ A Γ₂ ⊢ ` "z" ⦂ A
------------------ ⊢` ------------------------------------- _·_
Γ₂ ⊢ ` "s" ⦂ A ⇒ A Γ₂ ⊢ ` "s" · ` "z" ⦂ A
---------------------------------------------- _·_
Γ₂ ⊢ ` "s" · (` "s" · ` "z") ⦂ A
-------------------------------------------- ⊢ƛ
Γ₁ ⊢ ƛ "z" ⇒ ` "s" · (` "s" · ` "z") ⦂ A ⇒ A
------------------------------------------------------------- ⊢ƛ
Γ ⊢ ƛ "s" ⇒ ƛ "z" ⇒ ` "s" · (` "s" · ` "z") ⦂ (A ⇒ A) ⇒ A ⇒ A
where `∋s` and `∋z` abbreviate the two derivations:
------------------ Z
"s" ≢ "z" Γ₁ ∋ "s" ⦂ `ℕ ⇒ `ℕ
------------------------------ S ------------ Z
Γ₂ ∋ "s" ⦂ `ℕ ⇒ `ℕ Γ₂ ∋ "z" ⦂ `ℕ
---------------- Z
"s" ≢ "z" Γ₁ ∋ "s" ⦂ A ⇒ A
----------------------------- S ------------- Z
Γ₂ ∋ "s" ⦂ A ⇒ A Γ₂ ∋ "z" ⦂ A
and where ``Γ₁ = ∅ , s ⦂ `ℕ ⇒ `ℕ`` and ``Γ₂ = ∅ , s ⦂ `ℕ ⇒ `ℕ , z ⦂ `ℕ``.
In fact, `plusᶜ` also has this typing derivation if we replace `∅` by an
arbitrary context `Γ`, and `` `ℕ `` by an arbitrary type `A`, but the above
will suffice for our purposes.
and where `Γ₁ = Γ , s ⦂ A ⇒ A` and `Γ₂ = Γ , s ⦂ A ⇒ A , z ⦂ A`.
The typing derivation is valid for any `Γ` and `A`, for instance,
we might take `Γ` to be `∅` and `A` to be `` `ℕ ``.
Here is the above typing derivation formalised in Agda.
\begin{code}
Ch : Type → Type
Ch A = (A ⇒ A) ⇒ A ⇒ A
⊢twoᶜ : ∅ ⊢ twoᶜ ⦂ Ch `ℕ
⊢twoᶜ : ∀ {Γ A} → Γ ⊢ twoᶜ ⦂ Ch A
⊢twoᶜ = ⊢ƛ (⊢ƛ (⊢` ∋s · (⊢` ∋s · ⊢` ∋z)))
where
∋s = S ("s" ≠ "z") Z
@ -1179,10 +1178,10 @@ Ch A = (A ⇒ A) ⇒ A ⇒ A
Here are the typings corresponding to computing two plus two.
\begin{code}
⊢two : ∅ ⊢ two ⦂ `ℕ
⊢two : ∀ {Γ} → Γ ⊢ two ⦂ `ℕ
⊢two = ⊢suc (⊢suc ⊢zero)
⊢plus : ∅ ⊢ plus ⦂ `ℕ ⇒ `ℕ ⇒ `ℕ
⊢plus : ∀ {Γ} → Γ ⊢ plus ⦂ `ℕ ⇒ `ℕ ⇒ `ℕ
⊢plus = ⊢μ (⊢ƛ (⊢ƛ (⊢case (⊢` ∋m) (⊢` ∋n)
(⊢suc (⊢` ∋+ · ⊢` ∋m′ · ⊢` ∋n′)))))
where
@ -1195,6 +1194,9 @@ Here are the typings corresponding to computing two plus two.
⊢2+2 : ∅ ⊢ plus · two · two ⦂ `ℕ
⊢2+2 = ⊢plus · ⊢two · ⊢two
\end{code}
In contrast to our earlier examples, here we have typed `two` and `plus`
in an arbitrary context rather than the empty context; this makes it easy
to use them inside other binding contexts as well as at the top level.
Here the two lookup judgements `∋m` and `∋m′` refer to two different
bindings of variables named `"m"`. In contrast, the two judgements `∋n` and
`∋n′` both refer to the same binding of `"n"` but accessed in different
@ -1203,7 +1205,7 @@ the second after "m" is bound in the successor branch of the case.
And here are typings for the remainder of the Church example.
\begin{code}
⊢plusᶜ : ∅ ⊢ plusᶜ ⦂ Ch `ℕ ⇒ Ch `ℕ ⇒ Ch `ℕ
⊢plusᶜ : ∀ {Γ A} → Γ ⊢ plusᶜ ⦂ Ch A ⇒ Ch A ⇒ Ch A
⊢plusᶜ = ⊢ƛ (⊢ƛ (⊢ƛ (⊢ƛ (⊢` ∋m · ⊢` ∋s · (⊢` ∋n · ⊢` ∋s · ⊢` ∋z)))))
where
∋m = S ("m" ≠ "z") (S ("m" ≠ "s") (S ("m" ≠ "n") Z))
@ -1211,14 +1213,11 @@ And here are typings for the remainder of the Church example.
∋s = S ("s" ≠ "z") Z
∋z = Z
⊢sucᶜ : ∅ ⊢ sucᶜ ⦂ `ℕ ⇒ `ℕ
⊢sucᶜ : ∀ {Γ} → Γ ⊢ sucᶜ ⦂ `ℕ ⇒ `ℕ
⊢sucᶜ = ⊢ƛ (⊢suc (⊢` ∋n))
where
∋n = Z
⊢2ᶜ : ∅ ⊢ twoᶜ · sucᶜ · `zero ⦂ `ℕ
⊢2ᶜ = ⊢twoᶜ · ⊢sucᶜ · ⊢zero
⊢2+2ᶜ : ∅ ⊢ plusᶜ · twoᶜ · twoᶜ · sucᶜ · `zero ⦂ `ℕ
⊢2+2ᶜ = ⊢plusᶜ · ⊢twoᶜ · ⊢twoᶜ · ⊢sucᶜ · ⊢zero
\end{code}