parse tools in erlang

7/29/2019 Parse Tools in Erlang

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leex and yecc, Parse tools forErlang

Leaf Corcoran, 2011


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Overview of Erlang

Functional languageSingle assignment on variablesDynamic typingPattern matching: functions, conditionals, assignment

Excellent concurrency with message passingHigh fault tolerance, "Let it crash"Modules can be hot loaded

OTP standard library


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Overview of Erlang Syntax2323. % numbers

564 + 34.5.

Var = 34. % variablesVar = 22. % ERROR

{tuple, "Hello", 'World', '$money'}.

"hello" == [104, 101, 108, 108, 111].

[X || X


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Overview of Erlang Modules% stuff.erl

-module(stuff).-export([get_name/1]).

get_name({person, {name, Name}, _}) ->io:format("~w~n", Name);

get_name(_) ->error("Don't understand").

1> c(stuff).{ok,stuff}

2> stuff:get_name({person, {name, "Leaf"}, {age, 22}}).Leaf


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Overview of Erlang Misc% fibonacci

fib(1) -> 0;fib(2) -> 1;fib(N) when N > 0 -> fib(N-1) + fib(N-2).

% quicksort

qsort([Pivot, Rest]) ->qsort([ X || X


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Scanner/parser communicationThe scanner is a function that takes string and returns list of

tokens.The parser is a function that takes list of tokens (or afunction that generates tokens) and returns a parse tree.

leex: scanner generator

yecc: parser generator

They come with OTP, standard distribution.


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Scanner/parser communicationFor Yecc, the list of tokens is a list of tuples containing token,

line, and optionally a value.

scanner:scan("(5 + 2) - 3").

[{'(', 1},{number, 1, 5},{'+', 1},{number, 1, 2},{')', 1},{'-', 1},

{number, 1, 3},{'$end', 1}

]


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yecc parser generatorLALR-1 Parser, based on same algorithm as yacc.

Matches against patterns of Erlang atoms, and runsassociated action.

Action is Erlang code.Evaluate directly, eg. to make a simple calculator,Can build tree nodes.

Has access to matched tokens using $ syntax.Has following sections

HeaderTerminals, Nonterminals

Grammar rulesErlang code

Creates a module that exports:parse/1parse_and_scan/1


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leex scanner generatorMaps regular expressions to Erlang code.

Result of code determines whether to generate token,skip data, or throw an error

Can also push characters back on streamHas three sections:

Definitions

RulesErlang code

Creates a module that exports:string/1

token/2, tokens/2


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Writing an interpreterEvaluate tokens by pattern matching on their type.

Pass environment as function argument.


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Message passingVery cheap to create processes, no shared data.

Pass around PID to keep track of processes.Sent messages are stored in mailbox.Receive block lets a process pattern match on messages.

-module(server).

-export([loop/0]).

loop() ->receive

{print, Msg} -> io:format("~p~n", [Msg])end,

loop().

1> Pid = spawn(server, loop, []).

2> Pid ! {print, "Hello world!"}.


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ConclusionError messages aren't good

SometimesThe grammar and parser can be verbose.Small learning curve.Documentation is decent but lacking.

Download code and presentation at: http://leafo.net/erlang
http://leafo.net/erlang


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Things to check outReia

http://reia-lang.org/Ruby-like scripting language that runs on the Erlang virtualmachine.Efenehttp://marianoguerra.com.ar/efene/

Programming language with C-like syntax that runs onErlang platform.Erjanghttp://www.erjang.org/

JVM implementation of Erlang virtual machine.
http://www.erjang.org/http://marianoguerra.com.ar/efene/http://www.erjang.org/http://marianoguerra.com.ar/efene/http://reia-lang.org/

parse tools in erlang

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