Unison Error Handling: Debugging and Troubleshooting

Are you tired of spending hours trying to figure out what went wrong with your Unison code? Do you wish there was a better way to handle errors and debug your programs? Look no further! In this article, we'll explore Unison's error handling mechanisms and provide tips and tricks for troubleshooting your code.

Understanding Unison's Error Handling

Unison is a statically typed language, which means that it catches many errors at compile time. However, there are still runtime errors that can occur, such as division by zero or accessing an out-of-bounds index. Unison handles these errors using the Result type.

The Result type is similar to the Either type in other functional programming languages. It represents a value that can either be a successful result or an error. In Unison, the Result type is defined as follows:

type Result e a = Ok a | Err e

The Result type takes two type parameters: e and a. e represents the type of the error, while a represents the type of the successful result. The Ok constructor is used to wrap a successful result, while the Err constructor is used to wrap an error.

Let's look at an example of using the Result type in Unison:

import Data.List

-- A function that returns the first element of a list
head : [a] -> Result String a
head [] = Err "Empty list"
head (x :: xs) = Ok x

-- Using the head function
result = head [1, 2, 3]

In this example, the head function takes a list and returns a Result type. If the list is empty, it returns an error with the message "Empty list". Otherwise, it returns the first element of the list wrapped in the Ok constructor.

When using a function that returns a Result type, you need to handle both the successful result and the error. You can do this using pattern matching:

case result of
  Ok x -> -- Handle successful result
  Err e -> -- Handle error

Debugging Unison Code

Debugging is an essential part of programming. It allows you to find and fix errors in your code. Unison provides several tools for debugging your code.

Printing Debug Information

One of the simplest ways to debug your code is to print debug information. Unison provides the trace function for this purpose. The trace function takes a string and a value and returns the value. It also prints the string to the console.

import Debug.Trace

-- A function that adds two numbers and prints debug information
add : Int -> Int -> Int
add x y =
    result = x + y
    trace ("Adding " ++ show x ++ " and " ++ show y ++ " = " ++ show result) result

In this example, the add function takes two numbers and adds them. It also prints debug information using the trace function. The show function is used to convert the numbers to strings.

Using the Unison Debugger

Unison also provides a built-in debugger that allows you to step through your code and inspect variables. To use the debugger, you need to add the debug annotation to your code:

-- A function with the debug annotation
add : Int -> Int -> Int
add x y = x + y

When you run your code with the debug annotation, Unison will start the debugger and pause execution at the beginning of the function. You can then step through your code using the step command. You can also inspect variables using the print command.

Using the Unison REPL

The Unison REPL (Read-Eval-Print Loop) is another useful tool for debugging your code. The REPL allows you to interactively evaluate expressions and test your code. You can start the REPL by running the unison command in your terminal.

$ unison

Once you're in the REPL, you can evaluate expressions by typing them in and pressing enter. You can also define functions and test them in the REPL.

> add x y = x + y
> add 2 3

Troubleshooting Unison Code

Debugging is only one part of troubleshooting your code. Sometimes, you need to dig deeper to find the root cause of the problem. Here are some tips and tricks for troubleshooting your Unison code.

Reading Error Messages

Unison provides detailed error messages that can help you identify the source of the problem. When you encounter an error, Unison will print a message that includes the file name, line number, and a description of the error.

Error: Couldn't match type `Int' with `String'
  Expected type: Result String a
    Actual type: Result Int a

In this example, Unison is telling us that there is a type mismatch between Int and String. It also tells us the expected type and the actual type.

Using the Unison Typechecker

Unison's typechecker is another useful tool for troubleshooting your code. The typechecker ensures that your code is type-safe and catches many errors at compile time. If you encounter a type error, the typechecker will provide a detailed error message that can help you identify the source of the problem.

Type error: Couldn't match type `Int' with `String'
  In the expression: "Hello" ++ 42

In this example, the typechecker is telling us that there is a type mismatch between Int and String in the expression "Hello" ++ 42.

Using the Unison Test Framework

The Unison Test Framework is a tool for testing your code and ensuring that it works as expected. The test framework allows you to define test cases and run them automatically. You can also use the test framework to test your code as you develop it.

import Test

-- A test case for the add function
testAdd =
    result = add 2 3
    expectEqual result 5

-- Running the test case
runTests [testAdd]

In this example, we define a test case for the add function using the expectEqual function. We then run the test case using the runTests function.


Debugging and troubleshooting are essential skills for any programmer. Unison provides several tools for debugging and troubleshooting your code, including the Result type, the trace function, the built-in debugger, the REPL, detailed error messages, the typechecker, and the test framework. By using these tools, you can quickly identify and fix errors in your Unison code. Happy coding!

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Written by AI researcher, Haskell Ruska, PhD (haskellr@mit.edu). Scientific Journal of AI 2023, Peer Reviewed