When debugging, it is often helpful to output the values of expressions and variables. This can be challenging in a functional language like Flix because print is impure, so:

def myAdd(x: Int32): Int32 =
    println("myAdd called with: ${x}");
    x + 1

Results in an error:

Impure function declared as pure.


Flix provides a special-case debug function which has the same signature as the identity fuction:

def debug(x: a): a

Note: debug isn't a true function, which means that there are a few limitations on how it can be used: see Limitations for details.

Because debug appears to be pure, it can be used anywhere within Flix, including pure functions:

def myAdd(x: Int32): Int32 =
    debug("myAdd called with: ${x}");
    x + 1

And because it returns its argument, it can be inserted within expressions without modifying their behaviour. For example let statements:

let x = debug(1 + 1);

For-yield expressions:

for(i <- List.range(0, 10);
    j <- debug(List.range(i, 10)))
    yield (i, j)

Or pipelines:

DelayList.from(42) |> -> x + 10) |> debug |> DelayList.take(10)

Output format

Debug output is not generated with toString. Instead debug prints the internal Flix datastructure. For example debug(1 :: 2 :: Nil) outputs:

Cons(1, Cons(2, Nil))

This means that debug can output any Flix datastructure, whether or not it implements the ToString class.

You can bypass this by calling toString yourself, or using string interpolation:


The debug format is available within string interploation by using %{...}:

println("Internally, lists look like this: %{1 :: 2 :: Nil}")

Source location and expression

Flix provides two additional variants of debug: debug! and debug!!. The first displays the source location from which debug was called, and the second both the source location and the source code of the expression passed to debug!!


  • Although debug appears to be pure, it is not, and should not be used in production code. Future versions of Flix will enforce this requirement by raising an error if debug appears in production builds.
  • The location and expression output by debug! and debug!! are not guaranteed to be correct if they're used as values (e.g. within a pipeline).
  • Because debug appears to be pure, it can be optimised away, for example if the value is not used:
    let _ = debug(...)