Skip to main content



The documentation that you're reading is a design document where most of the features you're reading are yet to be implemented. Check the Note on the Docs

NeoHaskell promotes the usage of variables that do not change over time. These kinds of variables are named constants.

In most of programming languages, what you expect is to use regular variables that can change, and then only if you think that some variable should not change, you create a constant. In NeoHaskell, the approach is the opposite: you should use constants by default, and only if you think that some variable should change, you create a mutable variable (we will talk about mutable variables in later sections).

In software development, mutable variables introduce cognitive overhead because developers must track changes to their values throughout the code, which can lead to unpredictable side effects and bugs. On the other hand, constants offer clarity and predictability. When you see a constant, you instantly know its value will remain consistent everywhere, eliminating the mental effort of tracking potential changes and simplifying the code's comprehension.

Declaring Constants

Declaring constants is easy, just write the name of the constant, followed by an equals sign and the value of the constant:

neo> myConstant = 10

The above code declares a constant named myConstant with the value 10. You can now use it to perform operations:

neo> myConstant + 5

A Note on Naming Convention

Note how the constant name is written with the first letter in lowercase, and then for each word, the first letter is written in uppercase. This is called camelCase and is the naming convention in NeoHaskell for constants, variables and functions.

Another thing that might have caught your attention is that it is not written in all caps, like MY_CONSTANT. This is because in NeoHaskell, everything is a constant, and variables have special syntax to declare them as mutable. So, there is no need to write constants in all caps.

If you try to define a constant in all caps, you will get an error message:

neo> MY_CONSTANT = 10
It looks like you're trying to define a constant in all caps. In NeoHaskell, constants are defined in `camelCase`,
as names that start with uppercase letters are reserved for types.

Read more about constants in the docs:

They Are Really Constant, Tho

You might be wondering, what happens if I try to change the value of a constant? Let's try it:

neo> myConstant = 10
neo> myConstant
neo> myConstant = 20
neo> myConstant

Hey, you lied to me! You said that constants are constant, but I just changed the value of myConstant!

Well, not really. What you did was to create a new constant named myConstant with the value 20. The old constant named myConstant with the value 10 doesn't exist anymore.

Let's try doing one thing, incrementing the constant by one:

neo> myConstant = 10
neo> myConstant = myConstant + 1
neo> myConstant

-- oops! the REPL froze! 🥶

Nope, it's not your computer, the REPL just froze (you can press Ctrl + C to cancel the operation), and it is an expected behavior.

Why is this happening? What's happening here is that when defining a constant, the compiler is not really calculating the value on the right side of the equals sign, but instead, it is just storing the expression as it is.

When you try to use it (like when you write myConstant in the REPL), the compiler will calculate and store the value of the constant at that moment, as it is the first time that you are using it. This is called lazy evaluation, and we will go more in depth in the next section.