Frequently Asked Questions (FAQ)

Common questions and workarounds for limitations of the language

Imports relative to my top-level file are not working

All top-level imports are relative to the current working directory. For example, if you have a file located at ./foo/bar.dhall that tries to import ./foo/baz.dhall via a relative import:

$ cat ./foo/bar.dhall

./baz.dhall

$ cat ./foo/baz.dhall

… that relative import will not work correctly if you feed that file to a Dhall interpreter via standard input:

$ dhall < ./foo/bar.dhall
↳ ./baz.dhall

Error: Missing file ./baz.dhall

This is because the interpreter does not know that the string fed in via standard input originally came from ./foo/bar.dhall. Therefore, the interpreter cannot process the relative import correctly.

However, the relative import does work correctly if you pass the program using the --file flag, like this:

$ dhall --file ./foo/bar.dhall

Can I create a function with default values for function arguments?

The Dhall configuration language provides language support for completing a record with default-valued fields using the :: operator.

For example, in Python you can write:

def greet(greeting="Hello", name="John"):
    print("{0}, {1}!".format(greeting, name))

greet()
greet(greeting="Hola")
greet(name="Jane")
greet(greeting="Hola",name="Jane")

… which produces this result:

$ python greet.py
Hello, John!
Hola, John!
Hello, Jane!
Hola, Jane!

The Dhall equivalent of the above code would be:

let greet =
      \(args : { greeting : Text, name : Text }) ->
        "${args.greeting}, ${args.name}!"

let Greeting =
      { Type = { greeting : Text, name : Text }
      , default = { greeting = "Hello", name = "John" }
      }

in  ''
    ${greet Greeting::{=}}
    ${greet Greeting::{ greeting = "Hola" }}
    ${greet Greeting::{ name = "Jane" }}
    ${greet Greeting::{ greeting = "Hola", name = "Jane" }}
    ''

… which produces the same result:

$ dhall text --file ./greet.dhall
Hello, John!
Hola, John!
Hello, Jane!
Hola, Jane!

How does `dhall lint` differ from `dhall format`?

dhall lint:

Removes unused let bindings
Consolidates nested let bindings to use a multiple-let binding

See: Add support for let expressions with multiple let bindings
Fixes let bindings containing equivalences (≡) that were probably intended as assertions.

See: Add dhall lint support for fixing malformed assertions

How do I update nested fields in a record?

You can use the with keyword for this purpose:

let example = { coordinate = { x = 5, y = 3 }, element = "Hg" }

in  example
  with coordinate.x = example.coordinate.x + 1

See Add support for with keyword for more details.

Why do empty lists require a type annotation?

Dhall cannot infer a polymorphic type for an empty list because Dhall represents polymorphic values as functions of types, like this:

\(a : Type) -> [] : List a

If the compiler treated an empty list literal as syntactic short-hand for the above polymorphic function then you’d get the unexpected behavior where a list literal is a function if the list has 0 elements but not a function otherwise.

Does Dhall support user-defined recursive types?

No, but you can translate recursive code to non-recursive code by following this guide: How to translate recursive code to Dhall