Hell (Haskell shell): Year in Review
Hell is my little shell scripting language which is a thin layer1 over Haskell, using its syntax, its standard library and runtime, with a simpler static type system. You can find examples here. Over the year of 2024, I’ve written a few non-trivial shell scripts in Hell at work, and now a few colleagues have used it, too. Now I can use that to evaluate strengths and weaknesses of it as a tool. I c
God-mode for Emacs
A month ago I blogged about ways to reduce strenuous key presses in my Emacs use. I analyzed my runs of chords in Emacs, then speculated on the merits of exclusive vs mixed editing. Since then I wrote an Emacs mode called god-mode. It’s a mode that you toggle in and out of, and when you’re in it, all keys are implicitly prefixed with C- (among other helpful shortcuts). Over all, it’s been a resoun
Try.do for recoverable errors in Haskell
UPDATE 2021-01-02: I have since written a GHC compiler plugin to implement an alternative ?-based syntax for early return. I prefer that one than use of Try.do, because it doesn’t require any type magic or special instances, and the ? is more readable. UPDATE: I’ve added a follow-up post to this here, where I address some criticisms of this post. The first half of this post is here. Please read th
Fast Haskell: Competing with C at parsing XML
In this post we’re going to look at parsing XML in Haskell, how it compares with an efficient C parser, and steps you can take in Haskell to build a fast library from the ground up. We’re going to get fairly detailed and get our hands dirty. A new kid on the block A few weeks ago Neil Mitchell posted a blog post about a new XML library that he’d written. The parser is written in C, and the API is
Use the REPL, Luke
There was an online discussion about iteration times in Haskell and whether and why they are slow. For me, it’s not slow. I do all my Haskell development using a REPL. Here are some tips I wrote up in that discussion. Prepare for GHCi use The first thing you want to do before writing anything for your project is make sure you can load your code in the REPL; GHCi. Sometimes you have special configu
The path package
Here I’d like to announce and explain the motivations behind my path package. Motivation It was after working on a number of projects at FP Complete that use file paths in various ways. We used the system-filepath package, which was supposed to solve many path problems by being an opaque path type. It occurred to me that the same kind of bugs kept cropping up: Expected a path to be absolute but it
Existentials and the heterogenous list fallacy
An oft-stated argument against static typing is that heterogenous lists are unreasonably difficult to model. Why is static typing being so difficult? Why can’t it just be like dynamic typing? This is a specious argument. For example, in one article I read, I saw: In fact you can program heterogeneous lists in dependently typed languages, but it’s unreasonably complicated. Python makes no complaint
The constraint trick for instances
and thought, “Shenanigans! Why not just have this?” Me too! I only learned of this solution relatively recently, and I know experienced Haskellers who also only understood this recently or still don’t. Hence this quick write up. Here’s the thought process. We’re writing a trivial pretty printer and we’re using Writer. We write things like:
My Haskell tooling wishlist
I spend a lot of my time on Haskell tooling, both for my hobbies and my job. Almost every project I work on sparks a desire for another piece of tooling. Much of the time, I’ll follow that wish and take a detour to implement that thing (Fay, structured-haskell-mode, hindent, are some Haskell-specific examples). But in the end it means less time working on the actual domain problem I’m interested i
Measuring duration in Haskell
Happy new year, everyone. It’s a new year and time for new resolutions. Let’s talk about time. Specifically, measuring it in Haskell. A wrong solution How do you measure how long something takes in Haskell? Here’s a naive attempt: import Control.Exception import Data.Time main = do start <- getCurrentTime evaluate (sum [1 .. 1000000]) end <- getCurrentTime print (diffUTCTime end start) Inaccurate
Lucid: templating DSL for HTML
I’m not big on custom templating languages, for reasons I’ll write about another time. I prefer EDSLs. I preferred the xhtml package back when that was what everybody used. It looked like this: header << thetitle << "Page title" thediv noHtml ! [theclass "logo"] << "…" thediv noHtml ! [identifier "login"] Pretty line-noisy to read, write and hard to edit in a reasonable manner. Later, blaze-html b
shell-conduit: Write shell scripts in Haskell with Conduit
shell-conduit: Write shell scripts in Haskell with Conduit As part of my series of write-about-personal-projects, my latest obsession is writing shell scripts with Michael Snoyman’s Conduit. Here is my package, shell-conduit. It’s still in the experimental phase, but I don’t forsee any changes now for a while. Bash is evil I hate writing scripts in Bash. Until now, it was the easiest way to just w
Formatting in Haskell
This post is about the formatting package. What’s wrong with printf? The Text.Printf module is problematic simply because it’s not type-safe: λ> import Text.Printf λ> printf "" 2 *** Exception: printf: formatting string ended prematurely λ> printf "%s" 2 *** Exception: printf: bad formatting char 's' And it’s not extensible in the argument type. The PrintfType class does not export its methods. An
hindent: A Haskell indenter
A question of style In this post I’m going to use the word “style” to refer to the way code is printed in concrete terms. No changes in the code that would yield a different syntax tree are considered “style” here. What’s the deal with code style? Code style is important! If you’re a professional Haskell programmer, you’re working with Haskell code all day. The following things are affected by the
The Identity monad trick
Update: Got the link! The Trivial Monad. Thanks, John. Naturally, all interesting Haskell things eventually lead back to Dan Piponi. I heard about this from John Wiegley a while ago, but every time I recall it, I can’t remember how it goes, so I thought I’d write it down for myself. I think there’s a paper about it, but I can’t find it. Hopefully I’m recalling it correctly. The Identity monad tric
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Ghosts of Departed Proofs Convenience
Web engines in Haskell
hindent 5: One style to rule them all
Lucid 2.0: clearer than before
Teaching: A good (and bad) example