CS计算机代考程序代写 Haskell cse130

cse130 l https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
2. Evaluate body with param set to 10 !
Two kinds of Values
We now have two kinds of Values
v ::= n -- OLD
| -- 1. Plain Int (as before)
2. A function¡¯s ¡°code¡±: a pair of ¡°parameter¡± and ¡°body-expression¡±
data Value
= VInt Int -- OLD
| VCode Id Expr --
Evaluating Lambdas and Applications
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cse130
https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
eval :: Env -> Expr -> Value
-- OLD
-- NEW
Lets make sure our tests work properly!
exLam1 = ELet "incr" (ELam "x" (EBin Add (EVar "x") (ENum 1)))
(
EApp (EVar "incr") (ENum 10)
)
-- >>> eval [] exLam1
-- 11
QUIZ
What should the following evaluate to?
J
eval env (ENum n) = ???
eval env (EVar x) = ???
eval env (EBin op e1 e2) = ???
eval env (ELet x e1 e2) = ???
eval env (ELam x e) = ???
eval env (EApp e1 e2) = ???
letc=1 i in
iaf
letinc=x->x+c in
inc 10
(A) Error/Undefined (B) 10
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C
i
At C
11
1
2/16/21, 8:50 AM

cse130 https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
(C) 11 (D) 0 (E) 1
exLam2 = ELet "c" (ENum 1)
(ELet "incr" (ELam "x" (EBin Add (EVar "x") (EVar "
c")))
(
EApp (EVar "incr") (ENum 10)
) )
-- >>> eval [] exLam2
-- ???
QUIZ
And what should this expression evaluate to?
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WE

cse130 Quit
letc=1 in
letinc=x->x+c
https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
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in
i0nc 10 (A) Error/Undefined
(B) 110 a
(C) 11
localscope eustatic
formal hay letc=100
in
oral produce
33
The ¡°Immutability Principle¡±
A function¡¯s behavior should never change
A function must always return the same output for a given input
Why?
How to charge to

cse130 https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
> myFunc 10 0
> myFunc 10 10
Oh no! How to find the bug? Is it
In myFunc or
In a global variable or
In a library somewhere else or ...
My worst debugging nightmare
Colbert ¡°Immutability Principle¡± (https://youtu.be/CWqzLgDc030?t=628)
The Immutability Principle ?
How does our eval work?
exLam3 = ELet "c" (ENum 1)
(
ELet "incr" (ELam "x" (EBin Add (EVar "x") (EVar "c")))
(
ELet "c" (ENum 100)
(
EApp (EVar "incr") (ENum 10)
) )
)
-- >>> eval [] exLam3
-- ???
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cse130 https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
Oops?
letc=1 in
letinc=x->x+c in
letc=100
in
= 1] <<< env inc 10 And so we get eval env (inc 10) -- [] -- ["c" := 1] -- ["inc" := , c := 1]
-- ["c" := 100, "inc" := eval ("x" := 10 : env) (x + c)
==>10+100
==> 110
Ouch.
Enforcing Immutability with Closures
How to enforce immutability principle inc 10 always returns 11 ?
Key Idea: Closures
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cse130 https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
At definition: Freeze the environment the function¡¯s value At call: Use the frozen environment to evaluate the body Ensures that inc 10 always evaluates to the same result!
letc=1 in
letinc=x->x+c
in
<<< frozenv = ["c" := 1] letc=100 in c>, "c" := 1]
inc 10
Now we evaluate
eval env (inc 10)
-- []
-- ["c" := 1]
-- ["inc" := , c := 1]
-- ["c" := 100, "inc" := eval ("x" := 10 : frozenv) (x + c) where frozenv = ["c" := 1]
==>10+1 ==> 1
tada!
Representing Closures
Lets change the Value datatype to also store an Env
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cse130 https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
data Value
= VInt Int -- OLD
| VClos Env Id Expr --
Evaluating Function Definitions
How should we fix the definition of eval for ELam ? eval :: Env -> Expr -> Value
eval env (ELam x e) = ???
Hint: What value should we bind incr to in our example above?
(Recall At definition freeze the environment the function¡¯s value)
Evaluating Function Calls
How should we fix the definition of eval for EApp ? eval :: Env -> Expr -> Value
eval env (EApp e1 e2) = ???
(Recall At call: Use the frozen environment to evaluate the body)
Hint: What value should we evaluate incr 10 to?
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cse130
Pei funEnv https://ucsd-cse130.github.io/wi21/lectures/05-environments.html funparam
KfenBody
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aL
1. Evaluate incr to get
2. Evaluate 10 to get 10
O
Cte x + c in x:=10 : frozenv IT
3. Evalua
Let¡¯s generalize that recipe!
1. Evaluate e1 to get 2. Evaluate e2 to get v2
3. Evaluate body in param := v2 : frozenv
Immutability Achieved
Lets put our code to the test!
exLam3 = ELet "c" (ENum 1)
(
ELet "incr" (ELam "x" (EBin Add (EVar "x") (EVar "c")))
(
ELet "c" (ENum 100)
(
EApp (EVar "incr") (ENum 10)
) )
)
-- >>> eval [] exLam3
-- ???
QUIZ
What should the following evaluate to?
Jez argvaluetextenvefenparan
argual

cse130
https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
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letadd=x->(y->x+y) in
let add10 = add 10 in
let add20 = add 20
30
Functions Returning Functions Achieved!
exLam4 = ...
-- >>> eval [] exLam4
TODO
Practice
What should the following evaluate to?
letadd=x->(y->x+y) in
let add10 = add 10 in
let doTwice = f -> (x -> f (f x)) in
doTwice add10 100
TODO
11
Add 10 100
in
di85io
C
azzo9
(add10 100) + (add20 1000)
Functions Accepting Functions Achieved!
eval CT 120
extwice

cse130 https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
exLam5 = ...
-- >>> eval [] exLam4
The Nano Language
Features of Nano:
1. Arithmetic expressions [done]
A defface Ins
. Variables [done] 2I
FainD
3
. Let-bindings [done] 4. Functions [done]
5. Recursion
... You figure it out Hw4 ... :-)
(https://ucsd-cse130.github.io/wi21/feed.xml) (https://twitter.com/ranjitjhala)
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cse130 https://ucsd-cse130.github.io/wi21/lectures/05-environments.html
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