Download Variable Scoping Rules in Erlang

Variable Scoping Rules in Erlang
László Lövei, Zoltán Horváth, Tamás Kozsik,
Roland Király
Department of Programming Languages and Compilers
Faculty of Informatics
ELTE, Budapest, Hungary
Supported by Ericsson Hungary, ELTE CNL and ELTE IKKK*
* GVOP-3.2.2-2004-07-0005/3.0
Erlang Programming Language
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Functional programming language and runtime
environment developed by Ericsson
Designed to build distributed, reliable, soft realtime concurrent systems (telecommunication)
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No static type system
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Lightweight processes and message passing
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Highly dynamic nature
7th International Conference on Applied Informatics, January 28-31, 2007
Introduction to Refactoring
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Restructuring program code without altering its
external behaviour
Simple refactorings are used every day
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Widely used in OO programming
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Rename variables or types, split functions
Extract interface or subclass, pull up a method
Tool support is very useful
Refactoring in Erlang heavily depends on
understading variables
7th International Conference on Applied Informatics, January 28-31, 2007
Variables in Erlang
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Variables are local to
a function clause
longest([H|T]) ->
max(T, size(H)).
When the function is
run, a value is bound
to the variable
max([],
M) -> M;
max([H|T],M) ->
S = size(H),
if
S>M -> N=S;
not S>M -> N=M
end,
max(T, N).
The value cannot be
changed in the same
run of the function
error() ->
A = 1,
A = 2.
7th International Conference on Applied Informatics, January 28-31, 2007
Variable Scoping
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Scope: program part
affected by the
variable
Traditional definition:
from the binding
until the last usage
max([H|T],M) ->
S = size(H),
if
S>M -> N=S;
not S>M -> N=M
end,
max(T, N).
Formal semantics:
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Variable context: name to value mapping
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Expressions are evaluated in a context
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Pattern matching extends the context
7th International Conference on Applied Informatics, January 28-31, 2007
Rename Variable
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Condition: the new
name is not used
This affects the whole
function clause!
New definition:
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Scope ≡ fun.clause
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Variables are
contained in scopes
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Condition: the scope
does not contain the
new name
max([H|T],M) ->
S = size(H),
if
S>M -> N=S;
not S>M -> N=M
end,
max(T, N).
max([H|T],M) ->
S = size(H),
if
S>M -> New=S;
not S>M -> New=M
end,
max(T, New).
7th International Conference on Applied Informatics, January 28-31, 2007
Merge Expression Duplicates
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Same variable ≡
same scope
Where may a variable
be used?
Visibility:
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Expressions in the
scope of the variable
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After the binding
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Minus shadowing
constructs
max([H|T],M) ->
S = size(H),
if
S>M -> N=S;
not S>M -> N=M
end,
max(T, N).
max([H|T],M)
S=size(H),
if
Cond
not Cond
end,
max(T, N).
->
Cond=S>M,
-> N=S;
-> N=M
7th International Conference on Applied Informatics, January 28-31, 2007
Extract function
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Binding points: first
pattern matches for a
variable
Other occurrences
use the variable
Conditions:
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max([H|T],M) ->
S = size(H),
N = if
S>M -> S;
not S>M -> M
end,
max(T, N).
cmp(M,S) -> if S>M->S;
not S>M -> M end.
Bound variables are
not used elsewhere
Used, but not bound
variables become
parameters
max([H|T],M) ->
S = size(H),
N = cmp(M,S),
max(T, N).
7th International Conference on Applied Informatics, January 28-31, 2007
Eliminate variable
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Conditions:
long([H|T]) ->
S = size(H),
– Variables used in the
G = all(T, fun
expression are visible
(H)-> size(H)<S end),
at substitution points
if not G -> long(T);
G -> S end.
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Unique binding
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Constructs that limit
visibility:
long([H|T]) ->
G = all(T, fun
– Function expressions
(X)-> size(X)<size(H)
end),
– Unsafe variable
if not G -> long(T);
usage
G -> size(H) end.
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7th International Conference on Applied Informatics, January 28-31, 2007
Implementation
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These concepts can be implemented by lookup
tables that contain
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the scope for every variable
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visible variables for every expression
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binding points for every variable
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usage points for every variable
These lookup tables can be used directly to
implement the transformations
7th International Conference on Applied Informatics, January 28-31, 2007
The End
Thank you for your attention!
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7th International Conference on Applied Informatics, January 28-31, 2007