Slim “Sarah” Lim

UC Berkeley Computer Science

Notion Labs


slim [at] sarahlim [dot] com

Smiling woman with shoulder-length black hair, wearing glasses and a striped shirt, her arms folded
Eragon is the best compilers book


My name is Slim, and I am a computer scientist. Depending on my mood I am either a software engineer at Notion or a PhD student at UC Berkeley, advised by Sarah Chasins under the NSF Graduate Research Fellowship and UC Berkeley Chancellor’s Fellowship.

My research interests span programming languages, human factors, and learning sciences. I study programming languages as user interfaces: in short, how the syntactic and semantic properties of a language shape the way people reason about and write programs in the real world. For instance, I ask questions about the usability of rich type systems, designing (co)algebraic abstractions for UI programming, and how different evaluation strategies might promote or hinder program comprehension in educational settings. As both an academic and practitioner, my goal is to meaningfully improve the state of practice via theoretically compelling formalisms, substantiated by rigorous empirical methods and user-centered design.

Previously I collaborated with Ink & Switch on rich text CRDTs; helped build tools for personal computing as an early engineer at Notion; worked in the Early Product Development group at Khan Academy; and interned with the Human Experience and Design group at Microsoft Research Cambridge.

I received my BA in Computer Science from Northwestern University, concentrating in human-computer interaction and theoretical computer science. My studies were supported by scholarships from Google, Microsoft, Palantir, Box, Quip, and others, for which I am grateful, though these companies do not necessarily represent my views. During undergrad I was affiliated with the following research groups:

I have a lot of hobbies:

Auxiliary interests include browser engines, type systems, text editors, Nix, WebAssembly, Rust, Haskell, crossword puzzles, document preparation, coiling and soldering custom USB cables, the Nintendo Switch, ultralight backpacking, jazz, wilderness emergency medicine, and critical theory.

Here are a few of my 99 theses:


Nov 2022

Chrome 108 ships with hints for inactive CSS properties, based on our research with Ply.

Sep 2022

Peritext: A CRDT for Collaborative Rich Text Editing was accepted to CSCW 2022. This work is a collaboration with Geoffrey Litt, Martin Kleppmann, and Peter van Hardenberg.

Jun 2022

I’m back at OPLSS 2022 in Eugene!

I attended the Dagstuhl Seminar on Theories of Programming organized by Amy Ko, Thomas LaToza, David C. Shepherd, and Dag Sjøberg.

May 2022

Geoffrey Litt and I gave a talk on Peritext at the UC Santa Cruz Languages, Systems, and Data Seminar.

Jan 2022

I joined the Metamuse podcast to talk about rich text editing with Adam Wiggins and Mark McGranaghan.

I ghostwrote a blog post on designing the Notion public API.

Nov 2021

New essay with Geoffrey Litt, Martin Kleppmann, and Peter van Hardenberg: Peritext: A CRDT for Rich-Text Collaboration chronicles our efforts to create a working CRDT for rich text.

Oct 2021

Attending SPLASH in Chicago. It’s good to be back!

I ghostwrote a blog post about my team’s work sharding Postgres at Notion.

Aug 2021

I started my PhD at Berkeley and transitioned to a contracting role at Notion.

Jun 2021

Spending the summer with Ink & Switch, collaborating with Geoffrey Litt on CRDT-based approaches to rich text collaboration.

Mar 2021

I received an NSF Graduate Research Fellowship to support my forthcoming PhD.

Aug 2020

I’m co-organizing social sessions for the PL/HCI Swimmer School, along with Yanyan Ren and Zeeshan Lakhani.

Jun 2020

I’ve finally implemented inline mathematics in Notion! (See the release notes.) This was a very personally satisfying project, given my longstanding interests in mathematics and document preparation. We’re using the wonderful KaTeX library created by my alma mater, Khan Academy.

Apr 2020

I will be joining the UC Berkeley Computer Science PhD program in 2021, working with Sarah Chasins and supported by a Chancellor’s Fellowship. I’m very excited!

Jan 2020

Notion 2.7 is released, featuring search that actually works. Previously, Notion search was a major pain point: it was slow, inaccurate, and the interface was clunky. I spent the past several months rethinking and rewriting this feature from head to toe, and I’m really proud of what we came up with!

Oct 2019

Firefox 70 is released with Inactive CSS, a new inspection feature which highlights ineffective CSS properties arising from unsatisfied implicit dependencies. This work was directly inspired by Ply. Give it a try!

Sep 2019

Attending Papers We Love Conf and Strange Loop in St. Louis, through generous support from Project Alloy.

Aug 2019

Attending React Rally in Salt Lake City, where I gave an invited talk on WebAssembly entitled “All the memory safety of C combined with all the blazing speed of JavaScript” (slides). The title is a reference to James Iry's timeless blog post.

Jun 2019

Attending OPLSS in Eugene. Say hi if you’re around!

Apr 2019

New CSS inspection features inspired by our research are now available to test on Firefox Nightly, targeting Firefox 69 for a stable release!

Oct 2018

Attending UIST in Berlin, where our paper Ply: A Visual Web Inspector for Learning from Professional Webpages received Best Paper Honorable Mention. I presented this work (recording; slides).

Sep 2018

Attending ICFP, Strange Loop, and RacketCon in St. Louis, via PLMW.

Jul 2018

Attending Curry On! in Amsterdam.

Started an internship at MSR Cambridge.

Jun 2018

I graduated! I was honored to be named Outstanding Senior in Computer Science.

May 2017

Spoke at the Northwestern Big Ideas Forum, “How We Learn About Learning,” with professors Nell O’Rourke and David Uttal, and fellow undergrad Gabby Ashenafi.

Attending CHI in Denver. Ply won first place in the Student Research Competition!

Apr 2017

Received a Microsoft Tuition Scholarship for 2017-18.

Jan 2017

Ply: Visual Regression Pruning for Web Design Source Inspection is accepted to the CHI 2017 SRC.


Escapades in research and development, ranging from serious research projects to one-off diversions.

Peritext: A CRDT for Rich-Text Collaboration


Conflict-free Replicated Data Types (CRDTs) are commonly viewed as the holy grail of collaboration algorithms because they are mathematically well-behaved in comparison to approaches based on Operational Transforms (OT). Yet few CRDTs provide first-class support for rich text collaboration. We study the design principles for a rich text CRDT and present a prototype implementation, including algorithms for inserting and deleting text with formatting annotations. Collaboration with Geoffrey Litt, Martin Kleppmann, and Peter van Hardenberg.

RNG mechanics in Pokémon Sword and Shield


I wrote a tool for RNG manipulation in Pokémon Sword and Shield, written using wasm-bindgen. Users enter a seed for an active raid den and the tool calculates the corresponding stats for each possible encounter. Raid generation is implemented in Rust, which is much faster than other Web-based seed searching tools.

Ply: Visual Web Inspection

Delta Lab


CSS is syntactically straightforward, but has a steep learning curve and complicated semantics. Inspecting the source of existing webpages can help illustrate concepts, but such webpages are typically too complex to serve as useful learning materials. Drawing inspiration from prior research in both software engineering and the learning sciences, we present a novel web inspection tool and set of techniques for analyzing relevant CSS. We introduce the concept of implicit dependencies between CSS properties, which represent a major source of confusion for programmers. As a result of this work, Mozilla Firefox 70 ships with Inactive CSS to help identify and surface implicit dependencies within the Firefox Developer Tools. As of late 2022, Chrome 108 supports the same! Supervised by Haoqi Zhang and Nell O’Rourke. Honorable Mention Paper at UIST 2018, Berlin.

Tracing WebAssembly function calls


With Meg Grasse and mentorship from Nick Fitzgerald and Jim Blandy, we developed a proof-of-concept tool for instrumenting WebAssembly binaries written in Rust to log function calls at runtime.

Spelling correction with prefix tries


A more performant implementation of Peter Norvig's statistical spelling corrector, written in Go using prefix tries.

Theorems in Markdown


Pandoc is my single favorite piece of software. I use it to compile Markdown documents into slideshows, academic papers, course notes, blog posts, and more. Pandoc already handles LaTeX very well, but I wanted to typeset my notes with proper definition, theorem, and lemma environments using Markdown. This filter compiles definition lists into amsthm environments. It's written in Haskell.

Evaluating peer graders

Northwestern University


Existing models for peer grading borrow from the literature on crowdsourcing and forecasting, using a random variable to represent the score reported by a peer reviewer on a given submission. Most models attribute error in reporting solely to the skill of the peer reviewer, which unfairly penalizes peers who report noisy scores on unclear submissions. Building on the vancouver algorithm by de Alfaro and Shavlovsky, we use low-rank approximations to factor out reviewer skill and submission clarity. Supervised by Jason Hartline; paper in progress.

Visual regression pruning

Delta Lab


We introduce a visual significance heuristic for removing irrelevant CSS source code during web design reverse-engineering tasks. CHI 2017 Student Research Competition Winner, Denver, Colorado.

Guiding Web Inspection with Tutorial Keyword Frequency

Delta Lab


In order to bridge the gap between web design tutorials and real-world examples, we extend a web inspector to highlight CSS properties frequently mentioned across a given set of tutorials. Google Scholars’ Retreat 2016, Mountain View, California.


I was a teaching assistant every quarter beginning my sophomore year, sometimes for two courses at once. Terms marked with an asterisk (*) denote a head or sole teaching assistant role.