Download Foundations of Programming Languages

Foundations of Programming Languages
Closures and Thunks
Prof. Dr. Christoph Reichenbach
Fachbereich 12 / Institut für Informatik
14. November 2014
Partially Applied Functions
Haskell
add x =
let add2 y = x + y
in add2
add 2 3 --> 5
add 2 7 --> 9
2 / 10
Partially Applied Functions
Haskell
add x =
let add2 y = x + y
in add2
add 2 3 --> 5
add 2 7 --> 9
add 3
--> function: x 7→ x + 3
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Can pass ‘add 3’ as function parameter
Other languages: store ‘add 3’ in variable
How can we implement such a thing?
2 / 10
Naive implementation
add 3
3 / 10
Naive implementation
add 3
move $v0, $a0
addi $v0, 3
jreturn
3 / 10
Naive implementation
add 3
add 5
move $v0, $a0
addi $v0, 3
jreturn
move $v0, $a0
addi $v0, 5
jreturn
3 / 10
Naive implementation
add 3
add 5
add z
move $v0, $a0
addi $v0, 3
jreturn
move $v0, $a0
addi $v0, 5
jreturn
z not defined until run-time
Not a fully general implementation scheme
3 / 10
Closures
Idea: store function + environment:
hf , ei
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f : Code pointer
e: Environment
captures variables we know
4 / 10
Executing Closures
.text
heap
li
jal
move
; Create closure
$a0, 7
add
$s0, $v0
...
5 / 10
Executing Closures
.text
heap
7 (x)
ld
add
jreturn
li
jal
move
$v0, 8($a0)
$v0, $a1
; Create closure
$a0, 7
add
$s0, $v0
...
5 / 10
Executing Closures
.text
heap
ld
add
jreturn
7 (x)
$s0
li
jal
move
$v0, 8($a0)
$v0, $a1
; Create closure
$a0, 7
add
$s0, $v0
...
5 / 10
Executing Closures
.text
heap
ld
add
jreturn
7 (x)
$s0
li
jal
move
li
move
ld
jalr
$v0, 8($a0)
$v0, $a1
; Create closure
$a0, 7
add
$s0, $v0
...
; Invoke Closure
$a1, 35
$a0, $s0
$t0, 0($a0)
$t0
5 / 10
Thunks
I
I
Thunk: Closure that expects no parameters
Used to implement call-by-name
6 / 10
Thunks
I
I
Thunk: Closure that expects no parameters
Used to implement call-by-name
Some languages extend thunks with result caches
6 / 10
Thunks with Result Cache
.text
heap
2
sd
la
sd
jreturn
...
; computation
$v0, 8($a0)
$t0, quickret
$t0, 0($a0)
7 / 10
Thunks with Result Cache
heap
.text
value
...
; computation
$v0, 8($a0)
$t0, quickret
$t0, 0($a0)
sd
la
sd
jreturn
7 / 10
Thunks with Result Cache
heap
.text
value
...
; computation
$v0, 8($a0)
$t0, quickret
$t0, 0($a0)
sd
la
sd
jreturn
quickret:
ld
$v0, 8($a0)
jreturn
After first evaluation: result stored, code pointer
changed to fast read subroutine
7 / 10
Uses of Closures and Thunks
Closures:
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Partial application/currying (functional programming):
add 7
I
Passing subprogram parameters
Thunks:
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Pass-By-Name
Pass-By-Need (with updates)
8 / 10
Closures in the Wild
Python
def add(x):
def add2(y):
return x + y
return add2
C++
std :: function <int(int)> add(int i)
{ return [=] (int j) { return i + j; }; }
Java
static Function <Integer , Integer >
add(int x)
{ return y -> x + y; }
9 / 10
Summary
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Closure: hf , ei
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I
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Closures can be used like functions
Callers must use special operations
Thunk: Closure without parameters
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Subroutine pointer f
Environment e that captures local variables
May cache result
Call-by-name: implemented with uncached thunks
Call-by-need: implemented with cached thunks
10 / 10