This chapter clarifies all types.

To Pickup the overview more quickly they can be viewed in the following list.

Every decision and further specification are explained in the corresponding sub-chapters respectively.

List of all Types


BoolA value of type boolean can be either true or false
IntTODO: The classical integer type with 32-bit or 64-bit depending on the current system?
FloatA float with 64 bits?
StringA string of any unicode characters
PathPath referencing a location or a file. With no assumptions on the existence of that
NullThe value null

The Any type (upper boundary)

The Any type is often used to explicitly allow arbitrary values. However, the Any type is complex and doesn't add much value to a type system. Instead, we should use type variables whenever possible.

Interestingly there are two different Any types:

e.g. If we look at CUE-lang (which is also inspired by nix)

CUE defines the values bottom, or error, (denoted |) that is an instance of all types and top, or any, (denoted _) of which all types are an instance.

  • TOP any all types are an instance of that. You can imagine it as the TOP-most set, that includes every type. But no value has that type.

  • Bottom any which is an instance of all types. This is kind of the imaginary value that has the any type. Still, doesn't contain any value. Which could also be denoted: Never or Empty Type it is a type that is the subtype of any type.

The following is a nice quote from the Typescript world

The any type is so dangerous because it exists outside of the type tree. It is both a top and bottom type. Everything can be assigned to it and it can be assigned to everything else. ...

I think it might make most sense to define Any as TOP in our type system. This means it is the upper boundary of our type system. All types within the system are a subtype of Any.

Never (lower boundary)

I think it makes most sense to have a distinct Bottom type. Other type systems call this: Bottom any or Empty Type. This is the lower boundary of our type system. All types are a supertype of Never and that is true for all types that may eventualy exist in this type system.

The easiest way of thinking about Never is this example:

 # a -> Never
 f = x: abort "now";
 # Never <- f 42
 result = f 42;
 # result :: Never

A function f that takes Any argument and since it aborts the evaluation it returns Never.

Never might need its own chapter since it requires understanding lazyness in the language to determine which expression returns never during evaluation.

The Bool type

As the Bool type can only have two values (true/false)

The following is the definition of the Bool type

Bool :: true | false


T, U, ...; are placeholders for any types, those MUST be specifically declared on usage

[T]ListList of elements with type T eachList
{N::T}AttrSetAttrSet where member N references value of type TAttrSet
T->ULambdaA function that takes a single argument of type T and returns a value of type Ulambda

More details how to use complex types in the corresponding chapters


NumberInt {or} FloatThe Number is either of type Int or of type Float
Any?There is no Any type and it is explicitly prohibited to use the Any type. Use type variables instead if you want to allow variable type signatures.
StorePathPathThe StorePath is just a meaningful alias of the type Path
Derivation{ ... }TODO: Derivation is just a special AttrSet.
PackageDerivation {or} {...}TODO: Package is either a Derivation or a special AttrSet with name xy in it.

used operators are defined in the operators chapter