Slide 1
Slide 1 text
Introduction to
Concurrency in Haskell
Abhinav Sarkar
Functional Conf / Sep 12, 2015
Slide 2
Slide 2 text
I Am
Abhinav Sarkar
@abhin4v
@nilenso
Slide 3
Slide 3 text
Haskell
Pure functional programming
Strict type system
Immutable data structures
Good for concurrent programs
Slide 4
Slide 4 text
Real World Scenarios
Threads
Locks/Synchronization
Inter thread communication
Asynchronous exceptions
Transactions
Slide 5
Slide 5 text
Multiuser Chat
Slide 6
Slide 6 text
Features
Users
Channels
Message and Tell
Slide 7
Slide 7 text
Feature Plan
Accept user connections
User to user chat
User quitting and timeouts (maybe)
Channels: joining, leaving and chatting
Slide 8
Slide 8 text
Start!
Slide 9
Slide 9 text
m
o
d
u
l
e L
i
n
k
.
T
y
p
e
s w
h
e
r
e
d
a
t
a U
s
e
r = U
s
e
r { u
s
e
r
N
a
m
e :
: S
t
r
i
n
g }
d
e
r
i
v
i
n
g (
S
h
o
w
, E
q
, O
r
d
)
d
a
t
a C
l
i
e
n
t = C
l
i
e
n
t {
c
l
i
e
n
t
U
s
e
r :
: U
s
e
r
, c
l
i
e
n
t
H
a
n
d
l
e :
: H
a
n
d
l
e
} d
e
r
i
v
i
n
g (
S
h
o
w
, E
q
)
d
a
t
a S
e
r
v
e
r = S
e
r
v
e
r {
s
e
r
v
e
r
U
s
e
r
s :
: M
V
a
r (
M
a
p
.
M
a
p U
s
e
r C
l
i
e
n
t
)
}
Slide 10
Slide 10 text
Accept User Connections
Slide 11
Slide 11 text
m
o
d
u
l
e L
i
n
k
.
S
e
r
v
e
r w
h
e
r
e
i
m
p
o
r
t L
i
n
k
.
T
y
p
e
s
r
u
n
S
e
r
v
e
r :
: I
n
t -
> I
O (
)
r
u
n
S
e
r
v
e
r p
o
r
t = w
i
t
h
S
o
c
k
e
t
s
D
o $ d
o
h
S
e
t
B
u
f
f
e
r
i
n
g s
t
d
o
u
t L
i
n
e
B
u
f
f
e
r
i
n
g
s
e
r
v
e
r <
- n
e
w
S
e
r
v
e
r
s
o
c
k <
- l
i
s
t
e
n
O
n . P
o
r
t
N
u
m
b
e
r . f
r
o
m
I
n
t
e
g
r
a
l $ p
o
r
t
p
r
i
n
t
f "
L
i
s
t
e
n
i
n
g o
n p
o
r
t %
d
\
n
" p
o
r
t
f
o
r
e
v
e
r $ d
o
(
h
a
n
d
l
e
, h
o
s
t
, p
o
r
t
'
) <
- a
c
c
e
p
t s
o
c
k
p
r
i
n
t
f "
A
c
c
e
p
t
e
d c
o
n
n
e
c
t
i
o
n f
r
o
m %
s
: %
s
\
n
" h
o
s
t (
s
h
o
w p
o
r
t
'
)
f
o
r
k
I
O $ c
o
n
n
e
c
t
C
l
i
e
n
t s
e
r
v
e
r h
a
n
d
l
e `
f
i
n
a
l
l
y
` h
C
l
o
s
e h
a
n
d
l
e
Slide 12
Slide 12 text
Threads
Green threads
Non blocking IO
Memory efficient
-threaded ghc option
Slide 13
Slide 13 text
MVar
Mutable variable
As a container for shared state
As a blocking primitive
For locks/synchronization
Slide 14
Slide 14 text
c
o
n
n
e
c
t
C
l
i
e
n
t :
: S
e
r
v
e
r -
> H
a
n
d
l
e -
> I
O (
)
c
o
n
n
e
c
t
C
l
i
e
n
t s
e
r
v
e
r h
a
n
d
l
e = d
o
h
S
e
t
N
e
w
l
i
n
e
M
o
d
e h
a
n
d
l
e u
n
i
v
e
r
s
a
l
N
e
w
l
i
n
e
M
o
d
e
h
S
e
t
B
u
f
f
e
r
i
n
g h
a
n
d
l
e L
i
n
e
B
u
f
f
e
r
i
n
g
r
e
a
d
N
a
m
e
w
h
e
r
e
Slide 15
Slide 15 text
r
e
a
d
N
a
m
e :
: I
O (
)
r
e
a
d
N
a
m
e = d
o
n
a
m
e <
- h
G
e
t
L
i
n
e h
a
n
d
l
e
i
f n
u
l
l n
a
m
e
t
h
e
n r
e
a
d
N
a
m
e
e
l
s
e d
o
l
e
t u
s
e
r = U
s
e
r n
a
m
e
o
k <
- c
h
e
c
k
A
d
d
C
l
i
e
n
t s
e
r
v
e
r u
s
e
r h
a
n
d
l
e
c
a
s
e o
k o
f
N
o
t
h
i
n
g -
> d
o
h
P
r
i
n
t
f h
a
n
d
l
e
"
T
h
e n
a
m
e %
s i
s i
n u
s
e
, p
l
e
a
s
e c
h
o
o
s
e a
n
o
t
h
e
r
\
n
" n
a
m
e
r
e
a
d
N
a
m
e
J
u
s
t c
l
i
e
n
t -
>
r
u
n
C
l
i
e
n
t s
e
r
v
e
r c
l
i
e
n
t
`
f
i
n
a
l
l
y
` r
e
m
o
v
e
C
l
i
e
n
t s
e
r
v
e
r u
s
e
r
Slide 16
Slide 16 text
c
h
e
c
k
A
d
d
C
l
i
e
n
t :
: S
e
r
v
e
r -
> U
s
e
r -
> H
a
n
d
l
e -
> I
O (
M
a
y
b
e C
l
i
e
n
t
)
c
h
e
c
k
A
d
d
C
l
i
e
n
t S
e
r
v
e
r {
.
.
} u
s
e
r
@
U
s
e
r {
.
.
} h
a
n
d
l
e =
m
o
d
i
f
y
M
V
a
r s
e
r
v
e
r
U
s
e
r
s $ \
c
l
i
e
n
t
M
a
p -
>
i
f M
a
p
.
m
e
m
b
e
r u
s
e
r c
l
i
e
n
t
M
a
p
t
h
e
n r
e
t
u
r
n (
c
l
i
e
n
t
M
a
p
, N
o
t
h
i
n
g
)
e
l
s
e d
o
c
l
i
e
n
t <
- C
l
i
e
n
t u
s
e
r h
a
n
d
l
e
p
r
i
n
t
f "
N
e
w u
s
e
r c
o
n
n
e
c
t
e
d
: %
s
\
n
" u
s
e
r
N
a
m
e
r
e
t
u
r
n (
M
a
p
.
i
n
s
e
r
t u
s
e
r c
l
i
e
n
t c
l
i
e
n
t
M
a
p
, J
u
s
t c
l
i
e
n
t
)
r
e
m
o
v
e
C
l
i
e
n
t :
: S
e
r
v
e
r -
> U
s
e
r -
> I
O (
)
r
e
m
o
v
e
C
l
i
e
n
t S
e
r
v
e
r {
.
.
} u
s
e
r =
m
o
d
i
f
y
M
V
a
r
_ s
e
r
v
e
r
U
s
e
r
s $ r
e
t
u
r
n . M
a
p
.
d
e
l
e
t
e u
s
e
r
Slide 17
Slide 17 text
User to User Chat
Slide 18
Slide 18 text
m
o
d
u
l
e L
i
n
k
.
T
y
p
e
s w
h
e
r
e
d
a
t
a C
l
i
e
n
t = C
l
i
e
n
t {
c
l
i
e
n
t
U
s
e
r :
: U
s
e
r
, c
l
i
e
n
t
H
a
n
d
l
e :
: H
a
n
d
l
e
, c
l
i
e
n
t
C
h
a
n :
: C
h
a
n M
e
s
s
a
g
e
}
d
a
t
a M
e
s
s
a
g
e = M
s
g U
s
e
r S
t
r
i
n
g
d
e
r
i
v
i
n
g (
S
h
o
w
, E
q
)
Slide 19
Slide 19 text
m
o
d
u
l
e L
i
n
k
.
P
r
o
t
o
c
o
l w
h
e
r
e
i
m
p
o
r
t L
i
n
k
.
T
y
p
e
s
p
a
r
s
e
C
o
m
m
a
n
d :
: S
t
r
i
n
g -
> M
a
y
b
e M
e
s
s
a
g
e
p
a
r
s
e
C
o
m
m
a
n
d c
o
m
m
a
n
d = c
a
s
e w
o
r
d
s c
o
m
m
a
n
d o
f
"
M
S
G
" : u
s
e
r
N
a
m
e : m
s
g -
>
J
u
s
t $ M
s
g (
U
s
e
r u
s
e
r
N
a
m
e
) (
u
n
w
o
r
d
s m
s
g
)
_ -
> N
o
t
h
i
n
g
f
o
r
m
a
t
M
e
s
s
a
g
e :
: M
e
s
s
a
g
e -
> S
t
r
i
n
g
f
o
r
m
a
t
M
e
s
s
a
g
e (
M
s
g u
s
e
r m
s
g
) =
p
r
i
n
t
f "
M
S
G %
s %
s
" (
u
s
e
r
N
a
m
e u
s
e
r
) m
s
g
Slide 20
Slide 20 text
Chans
Unbounded blocking queues
Slide 21
Slide 21 text
r
u
n
C
l
i
e
n
t :
: S
e
r
v
e
r -
> C
l
i
e
n
t -
> I
O (
)
r
u
n
C
l
i
e
n
t S
e
r
v
e
r {
.
.
} C
l
i
e
n
t {
.
.
} = f
o
r
e
v
e
r $ d
o
r <
- t
r
y $ r
a
c
e r
e
a
d
C
o
m
m
a
n
d r
e
a
d
M
e
s
s
a
g
e
c
a
s
e r o
f
L
e
f
t (
e :
: S
o
m
e
E
x
c
e
p
t
i
o
n
) -
>
p
r
i
n
t
f "
E
x
c
e
p
t
i
o
n
: %
s
\
n
" (
s
h
o
w e
)
R
i
g
h
t c
m -
> c
a
s
e c
m o
f
L
e
f
t m
c
o
m
m
a
n
d -
> c
a
s
e m
c
o
m
m
a
n
d o
f
N
o
t
h
i
n
g -
> p
r
i
n
t
f "
C
o
u
l
d n
o
t p
a
r
s
e c
o
m
m
a
n
d
\
n
"
J
u
s
t c
o
m
m
a
n
d -
> h
a
n
d
l
e
C
o
m
m
a
n
d c
o
m
m
a
n
d
R
i
g
h
t m
e
s
s
a
g
e -
> h
a
n
d
l
e
M
e
s
s
a
g
e m
e
s
s
a
g
e
w
h
e
r
e
Slide 22
Slide 22 text
r
e
a
d
C
o
m
m
a
n
d :
: I
O (
M
a
y
b
e M
e
s
s
a
g
e
)
r
e
a
d
C
o
m
m
a
n
d = d
o
c
o
m
m
a
n
d <
- h
G
e
t
L
i
n
e c
l
i
e
n
t
H
a
n
d
l
e
p
r
i
n
t
f "
<
%
s
>
: %
s
\
n
" (
u
s
e
r
N
a
m
e c
l
i
e
n
t
U
s
e
r
) c
o
m
m
a
n
d
r
e
t
u
r
n $ p
a
r
s
e
C
o
m
m
a
n
d c
o
m
m
a
n
d
r
e
a
d
M
e
s
s
a
g
e :
: I
O M
e
s
s
a
g
e
r
e
a
d
M
e
s
s
a
g
e = r
e
a
d
C
h
a
n c
l
i
e
n
t
C
h
a
n
s
e
n
d
M
e
s
s
a
g
e :
: M
e
s
s
a
g
e -
> I
O (
)
s
e
n
d
M
e
s
s
a
g
e m
e
s
s
a
g
e = w
r
i
t
e
C
h
a
n c
l
i
e
n
t
C
h
a
n
Slide 23
Slide 23 text
h
a
n
d
l
e
C
o
m
m
a
n
d :
: M
e
s
s
a
g
e -
> I
O (
)
h
a
n
d
l
e
C
o
m
m
a
n
d (
M
s
g u
s
e
r m
s
g
) =
c
l
i
e
n
M
a
p <
- r
e
a
d
M
V
a
r s
e
r
v
e
r
U
s
e
r
s
c
a
s
e M
a
p
.
l
o
o
k
u
p u
s
e
r c
l
i
e
n
t
M
a
p o
f
N
o
t
h
i
n
g -
>
p
r
i
n
t
f "
N
o s
u
c
h u
s
e
r
: %
s
\
n
" (
u
s
e
r
N
a
m
e u
s
e
r
)
J
u
s
t c
l
i
e
n
t -
>
s
e
n
d
M
e
s
s
a
g
e (
M
s
g c
l
i
e
n
t
U
s
e
r m
s
g
) c
l
i
e
n
t
h
a
n
d
l
e
M
e
s
s
a
g
e :
: M
e
s
s
a
g
e -
> I
O (
)
h
a
n
d
l
e
M
e
s
s
a
g
e = p
r
i
n
t
T
o
H
a
n
d
l
e c
l
i
e
n
t
H
a
n
d
l
e . f
o
r
m
a
t
M
e
s
s
a
g
e
Slide 24
Slide 24 text
r
a
c
e :
: I
O a -
> I
O b -
> I
O (
E
i
t
h
e
r a b
)
r
a
c
e i
o
a i
o
b = d
o
m <
- n
e
w
E
m
p
t
y
M
V
a
r
b
r
a
c
k
e
t (
f
o
r
k
I
O (
f
m
a
p L
e
f
t i
o
a
) (
p
u
t
M
V
a
r m
)
) k
i
l
l
T
h
r
e
a
d $ \
_ -
>
b
r
a
c
k
e
t (
f
o
r
k
I
O (
f
m
a
p R
i
g
h
t i
o
b
) (
p
u
t
M
V
a
r m
)
) k
i
l
l
T
h
r
e
a
d $ \
_ -
>
r
e
a
d
M
V
a
r m
Slide 25
Slide 25 text
MVar
For inter thread communication
Slide 26
Slide 26 text
Chatting in Channels
Slide 27
Slide 27 text
No content
Slide 28
Slide 28 text
No content
Slide 29
Slide 29 text
Software Transactional
Memory
Slide 30
Slide 30 text
Why Transactions?
Atomic changes to multiple variables
Read from multiple channels efficiently
Robustness in the presence of failure
Slide 31
Slide 31 text
m
o
d
u
l
e L
i
n
k
.
T
y
p
e
s w
h
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r { u
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N
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: !
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e }
d
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v
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g (
S
h
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w
, E
q
, O
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d
)
d
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a C
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l {
c
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h
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l
C
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n :
: T
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n M
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s
s
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g
e
}
Slide 32
Slide 32 text
d
a
t
a S
e
r
v
e
r = S
e
r
v
e
r {
s
e
r
v
e
r
U
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s :
: M
V
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r (
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M
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p U
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r C
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t
)
, s
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s :
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r (
M
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p
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M
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p C
h
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l
N
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m
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)
}
d
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a C
l
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n
t = C
l
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t {
c
l
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n
t
U
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r :
: !
U
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, c
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H
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C
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n M
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l
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a
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e (
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C
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e
s
s
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g
e
)
)
}
Slide 33
Slide 33 text
d
a
t
a M
e
s
s
a
g
e = M
s
g U
s
e
r S
t
r
i
n
g
| T
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l
l C
h
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l
N
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| J
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g (
S
h
o
w
, E
q
)
Slide 34
Slide 34 text
STM Monad
A monad of its own
Type system prevents doing IO
Slide 35
Slide 35 text
TVar
TVar: Transactional variable
n
e
w
T
V
a
r :
: a -
> S
T
M (
T
V
a
r a
)
r
e
a
d
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r :
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V
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r a -
> S
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M a
w
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r :
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r a -
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m
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l
y :
: S
T
M a -
> I
O a
r
e
t
r
y :
: S
T
M a
Slide 36
Slide 36 text
TChan
Transactional channels
Efficient merging
n
e
w
T
C
h
a
n :
: S
T
M (
T
C
h
a
n a
)
w
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C
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n :
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> a -
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o
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E
l
s
e :
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M a -
> S
T
M a
Slide 37
Slide 37 text
r
u
n
C
l
i
e
n
t :
: S
e
r
v
e
r -
> C
l
i
e
n
t -
> I
O (
)
r
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n
C
l
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t S
e
r
v
e
r {
.
.
} c
l
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@
C
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.
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} = d
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w
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Slide 38
Slide 38 text
r
u
n :
: I
O (
)
r
u
n = f
o
r
e
v
e
r $ d
o
r <
- t
r
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t
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m
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e :
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s
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Slide 39
Slide 39 text
h
a
n
d
l
e
M
e
s
s
a
g
e :
: M
e
s
s
a
g
e -
> I
O (
)
h
a
n
d
l
e
M
e
s
s
a
g
e (
J
o
i
n c
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n
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e
l
N
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m
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) = a
t
o
m
i
c
a
l
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y $ d
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c
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n
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D m
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Slide 40
Slide 40 text
High Level Design
Slide 41
Slide 41 text
High Level Design
Threads are cheap. Don't be afraid to launch new threads.
Non blocking IO. No need of event loops.
Small components connected through channels
Write small functions in STM and compose them
Slide 42
Slide 42 text
High Level Design
Structure programs like assembly lines
Processing functions read from and write to channels
Channels connect processing functions like conveyor belts
Channels can be duplicated and merged
Slide 43
Slide 43 text
That's All Folks
Slide 44
Slide 44 text
References
Parallel and Concurrent Programming in Haskell - Simon
Marlow
Real World Haskell - O'Sullivan, Stewart, Goerzen
Slide 45
Slide 45 text
https://github.com/abhin4v/link
https://tinyurl.com/introconchs
@abhin4v
[email protected]
Slide 46
Slide 46 text
Questions?