# Haskell

> Source: https://ollybritton.com/notes/programming/haskell/ · Updated: 2025-09-30 · Tags: notes, programming

Notes for learning Haskell. See also [redacted](https://ollybritton.com/404) and [Thinking Functionally With Haskell, Bird](https://ollybritton.com/notes/textbooks/thinking-functionally-with-haskell/).

## Project Ideas
- [ ] Write a Brainfuck interpreter in Haskell
- [ ] Write a tool like [LambdaCalc](https://lambdacalc.io/) that interprets lambda calculus expressions. It would take a source file, one with a file containing substitutions and a main source file, and would output the reduced lambda calculus expression. Would be fun for learning more about lambda calculus and Haskell at the same time.

## Exercism
##### `and` and `any` pattern
Need to check if every character is uppercase and there is at least one uppercase letter?

```haskell
import Data.Char (isUpper)

all isUpper input && any isUpper input
```

#### Check for `null` at the top
Because of lazy evaluation, checking for cases like the input being empty at the top of a function definition means that you don’t have to worry about issues later on down the line, like `last` not working. 

#### `Left` and `Right`
By convention, in functions where there can be a success or a failure:

- `Left` is used for failure
- `Right` is used for success

This is the other way around from Go!

#### Traverse
There’s a lot of stuff on the internet about “monads” and “applicatives” that I don’t understand yet related to `traverse`. In its use I’ve found so far, it lets you map over a list and return a new list if it’s successful on everything or return something else if it fails on any element.

#### `Map.FromListWith`
Creates a map from a list of (key, value) pairs and with list items with the same key being combined by a function `a -> a -> a`.

#### When working with splitting up a string
First of all, Haskell doesn’t have a function to split input up by a delimiter other than space (`words`). Second of all, it’s often a good idea to think “how could I change the input so that it’s easier to work with?”. This happened with my implementation of an `abbreviation` function which was super long-winded, and then I looked at the community solutions and then all modified the input text to make it much easier to work with.

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