Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
Monadic Parsing in Python
Search
Oleksii Kachaiev
June 06, 2014
Programming
13
7.2k
Monadic Parsing in Python
Oleksii Kachaiev
June 06, 2014
Tweet
Share
More Decks by Oleksii Kachaiev
See All by Oleksii Kachaiev
Counting HTTP with QUIC & HTTP/3
kachayev
2
250
Talking SQL to Strangers
kachayev
3
550
Counting HTTP: 0.9...3
kachayev
1
76
Managing Data Chaos in The World of Microservices
kachayev
3
640
Deep HTTP Dive Through Aleph & Netty
kachayev
6
3.7k
Keep Your Data Safe With Refined Types
kachayev
4
1.4k
Clojure at Attendify (2nd ed)
kachayev
5
1.6k
Clojure at Attendify
kachayev
4
330
Finagle & Clojure
kachayev
6
1.3k
Other Decks in Programming
See All in Programming
Duke on CRaC with Jakarta EE
ivargrimstad
1
150
SwiftDataのカスタムデータストアを試してみた
1mash0
0
150
Cloudflare Workersで進めるリモートMCP活用
syumai
7
920
Design Pressure
hynek
0
110
20250429 - CNTUG Meetup #67 / DevOps Taiwan Meetup #69 - Deep Dive into Tetragon: Building Runtime Security and Observability with eBPF
tico88612
0
180
プロダクトエンジニアのしごと 〜 受託 × 高難度を乗り越えるOptium開発 〜
algoartis
0
230
“技術カンファレンスで何か変わる?” ──RubyKaigi後の自分とチームを振り返る
ssagara00
0
120
CursorとDevinが仲間!?AI駆動で新規プロダクト開発に挑んだ3ヶ月を振り返る / A Story of New Product Development with Cursor and Devin
rkaga
5
1k
今話題のMCPサーバーをFastAPIでサッと作ってみた
yuukis
0
130
M5UnitUnified 最新動向 2025/05
gob
0
140
Beyond_the_Prompt__Evaluating__Testing__and_Securing_LLM_Applications.pdf
meteatamel
0
110
GitHub Copilot for Azureを使い倒したい
ymd65536
1
340
Featured
See All Featured
RailsConf 2023
tenderlove
30
1.1k
Building Adaptive Systems
keathley
41
2.5k
Refactoring Trust on Your Teams (GOTO; Chicago 2020)
rmw
34
3k
I Don’t Have Time: Getting Over the Fear to Launch Your Podcast
jcasabona
32
2.3k
How To Stay Up To Date on Web Technology
chriscoyier
790
250k
Cheating the UX When There Is Nothing More to Optimize - PixelPioneers
stephaniewalter
280
13k
How GitHub (no longer) Works
holman
314
140k
10 Git Anti Patterns You Should be Aware of
lemiorhan
PRO
656
60k
Building a Scalable Design System with Sketch
lauravandoore
462
33k
Large-scale JavaScript Application Architecture
addyosmani
512
110k
Performance Is Good for Brains [We Love Speed 2024]
tammyeverts
10
800
Gamification - CAS2011
davidbonilla
81
5.3k
Transcript
Monadic Parsing in Python Alexey Kachayev, 2014
About me • CTO at Attendify.com • Erlang, Clojure, Go,
Haskell • Fn.py library author • CPython & Storm contributor
Find me •@kachayev •github.com/kachayev •kachayev <$> gmail.com
Topic
Will talk •What is "parsing(ers)" •Approaches •Monadic parsing from scratch
•More…
Will talk •Less about theory •Much more about practice
Won’t talk •What "monad" is •Why FP is cool (*)
* you’ll understand it by yourself
Parsing
Definition •Takes grammar •Takes input string (?) •Returns tree (??)
or an error
None
For PL creators only?
Tasks • Processing information from logs • Source code analysing
• DSLs • Protocols & data formats • … and more
Approaches
Production rule S → SS|(S)|()
Grammar block = ["const" ident "=" number {"," ident "="
number} ";"] ["var" ident {"," ident} ";"] {"procedure" ident ";" block ";"} statement ! expression = ["+"|"-"] term {("+"|"-") term} ! term = factor {("*"|"/") factor} ! factor = ident | number | "(" expression ")" ! . . . .
•Top-down / bottom-up •Predictive / Backtracking •LL(k), LALR, LR, CYK
and others In theory
Manually!
@ wikipedia
Manually •Simple to understand •Hard to maintain •Really boring
Can we do better?
What we have •Context-free grammars •Formal theory •Well-defined algorithms •Standard
grammar notation(s)
So…
Parser generator •1. Parse DSL notation •2. Generate parser code
•("any" language)
Parser generator •*PEG* •*Yacc* •ANTLR •… and tens more
Parser generator •Pros •many targeted languages •formalism •performance & optimisations
Parser generator •Cons •another language •bounded in features •"compiled-time" mostly
Can we do better?
Monadic parsers & combinators
Functional Pearls Monadic Parsing in Haskell @Graham Hutton, @Erik Meijer
Parsec MPC library for Haskell
Parsec •Monadic parser combinator(s) •Works even with context- sensitive, infinite
LA grammars •Tens of ports to other langs
None
The Big Idea
Simple type Parser = String → Tree
Compose? type Parser = String → (Tree, String)
Generalize? type Parser a = String → (a, String)
Errors? type Parser a = String → Maybe (a, String)
Or better… type Parser a = String → [(a, String)]
Let’s try…
Snippets: http://goo.gl/leQIEE
None
None
None
None
… and so?
Expressiveness •[] for error •[s1] for single (predictive) •[s1..sN] for
backtracking
First-class citizen
None
Skip anything…
Recognise digit
Combinators
RegExp •and: "abc" •or: "a | b | c" •Kleene
star: "a*"
Derives •a? = "" | a •a+ = aa* •a{2,3}
= aa | aaa
None
None
laziness is cool for this do you need backtracking?
How to use it?
None
None
Cool! but..
ugly ugly not readable
Enhancements •use generators for "laziness" •"combine" function •Scala-style methods •"delay"
method
fn.py Stream
None
[1,2,3,4,5] expr →"[" digit (","digit)* "]"
None
Interesting! but..
Is it enough?
In Haskell
Can I do this in Python?
… hm
Challenge accepted!
In Python
How?
Desugaring…
What?
WAT??? even more like
unit a → Parser a
bind Parser a → (a → Parser b) → Parser
b
lift (a → b) → (a → Parser b)
lifted Parser a → (a → b) → Parser b
WAT??? ok, looks cool, but
None
None
How to use
And even more..
Haskell-style
Do-notation
None
None
(define R 2) (define diameter (lambda (r) (* 2 r)))
None
None
Looks nice!
Mutability kills backtracking :(
And more •errors handling •backtracking control •performance
Links • "funcparselib" http://goo.gl/daidQY • "Monadic parsing in Haskell" http://goo.gl/gygNlM
• "Higher-Order functions for Parsing" http://goo.gl/c8VOIZ • "Parsec" http://goo.gl/bdnDZQ • "Parcon" http://goo.gl/CT06S5 • "Pyparsing" http://goo.gl/gmr2lQ • "You Could Have Invented Monadic Parsing" http://goo.gl/h0rnOQ
Learn Haskell For Great Good
Q/A thanks for your attention,