nixpkgs/lib/debug.nix

/**
  Collection of functions useful for debugging
  broken nix expressions.

  * `trace`-like functions take two values, print
    the first to stderr and return the second.
  * `traceVal`-like functions take one argument
    which both printed and returned.
  * `traceSeq`-like functions fully evaluate their
    traced value before printing (not just to “weak
    head normal form” like trace does by default).
  * Functions that end in `-Fn` take an additional
    function as their first argument, which is applied
    to the traced value before it is printed.
*/
{ lib }:
let
  inherit (lib)
    isList
    isAttrs
    substring
    attrValues
    concatLists
    const
    elem
    generators
    id
    mapAttrs
    trace;
in

rec {

  # -- TRACING --

  /**
    Conditionally trace the supplied message, based on a predicate.


    # Inputs

    `pred`

    : Predicate to check

    `msg`

    : Message that should be traced

    `x`

    : Value to return

    # Type

    ```
    traceIf :: bool -> string -> a -> a
    ```

    # Examples
    :::{.example}
    ## `lib.debug.traceIf` usage example

    ```nix
    traceIf true "hello" 3
    trace: hello
    => 3
    ```

    :::
  */
  traceIf =
    pred:
    msg:
    x: if pred then trace msg x else x;

  /**
    Trace the supplied value after applying a function to it, and
    return the original value.


    # Inputs

    `f`

    : Function to apply

    `x`

    : Value to trace and return

    # Type

    ```
    traceValFn :: (a -> b) -> a -> a
    ```

    # Examples
    :::{.example}
    ## `lib.debug.traceValFn` usage example

    ```nix
    traceValFn (v: "mystring ${v}") "foo"
    trace: mystring foo
    => "foo"
    ```

    :::
  */
  traceValFn =
    f:
    x: trace (f x) x;

  /**
    Trace the supplied value and return it.

    # Inputs

    `x`

    : Value to trace and return

    # Type

    ```
    traceVal :: a -> a
    ```

    # Examples
    :::{.example}
    ## `lib.debug.traceVal` usage example

    ```nix
    traceVal 42
    # trace: 42
    => 42
    ```

    :::
  */
  traceVal = traceValFn id;

  /**
    `builtins.trace`, but the value is `builtins.deepSeq`ed first.


    # Inputs

    `x`

    : The value to trace

    `y`

    : The value to return

    # Type

    ```
    traceSeq :: a -> b -> b
    ```

    # Examples
    :::{.example}
    ## `lib.debug.traceSeq` usage example

    ```nix
    trace { a.b.c = 3; } null
    trace: { a = <CODE>; }
    => null
    traceSeq { a.b.c = 3; } null
    trace: { a = { b = { c = 3; }; }; }
    => null
    ```

    :::
  */
  traceSeq =
    x:
    y: trace (builtins.deepSeq x x) y;

  /**
    Like `traceSeq`, but only evaluate down to depth n.
    This is very useful because lots of `traceSeq` usages
    lead to an infinite recursion.


    # Inputs

    `depth`

    : 1\. Function argument

    `x`

    : 2\. Function argument

    `y`

    : 3\. Function argument

    # Type

    ```
    traceSeqN :: Int -> a -> b -> b
    ```

    # Examples
    :::{.example}
    ## `lib.debug.traceSeqN` usage example

    ```nix
    traceSeqN 2 { a.b.c = 3; } null
    trace: { a = { b = {…}; }; }
    => null
    ```

    :::
  */
  traceSeqN = depth: x: y:
    let snip = v: if      isList  v then noQuotes "[…]" v
                  else if isAttrs v then noQuotes "{…}" v
                  else v;
        noQuotes = str: v: { __pretty = const str; val = v; };
        modify = n: fn: v: if (n == 0) then fn v
                      else if isList  v then map (modify (n - 1) fn) v
                      else if isAttrs v then mapAttrs
                        (const (modify (n - 1) fn)) v
                      else v;
    in trace (generators.toPretty { allowPrettyValues = true; }
               (modify depth snip x)) y;

  /**
    A combination of `traceVal` and `traceSeq` that applies a
    provided function to the value to be traced after `deepSeq`ing
    it.


    # Inputs

    `f`

    : Function to apply

    `v`

    : Value to trace
  */
  traceValSeqFn =
    f:
    v: traceValFn f (builtins.deepSeq v v);

  /**
    A combination of `traceVal` and `traceSeq`.

    # Inputs

    `v`

    : Value to trace

  */
  traceValSeq = traceValSeqFn id;

  /**
    A combination of `traceVal` and `traceSeqN` that applies a
    provided function to the value to be traced.


    # Inputs

    `f`

    : Function to apply

    `depth`

    : 2\. Function argument

    `v`

    : Value to trace
  */
  traceValSeqNFn =
    f:
    depth:
    v: traceSeqN depth (f v) v;

  /**
    A combination of `traceVal` and `traceSeqN`.

    # Inputs

    `depth`

    : 1\. Function argument

    `v`

    : Value to trace
  */
  traceValSeqN = traceValSeqNFn id;

  /**
    Trace the input and output of a function `f` named `name`,
    both down to `depth`.

    This is useful for adding around a function call,
    to see the before/after of values as they are transformed.


    # Inputs

    `depth`

    : 1\. Function argument

    `name`

    : 2\. Function argument

    `f`

    : 3\. Function argument

    `v`

    : 4\. Function argument


    # Examples
    :::{.example}
    ## `lib.debug.traceFnSeqN` usage example

    ```nix
    traceFnSeqN 2 "id" (x: x) { a.b.c = 3; }
    trace: { fn = "id"; from = { a.b = {…}; }; to = { a.b = {…}; }; }
    => { a.b.c = 3; }
    ```

    :::
  */
  traceFnSeqN = depth: name: f: v:
    let res = f v;
    in lib.traceSeqN
        (depth + 1)
        {
          fn = name;
          from = v;
          to = res;
        }
        res;


  # -- TESTING --

  /**
    Evaluates a set of tests.

    A test is an attribute set `{expr, expected}`,
    denoting an expression and its expected result.

    The result is a `list` of __failed tests__, each represented as
    `{name, expected, result}`,

    - expected
      - What was passed as `expected`
    - result
      - The actual `result` of the test

    Used for regression testing of the functions in lib; see
    tests.nix for more examples.

    Important: Only attributes that start with `test` are executed.

    - If you want to run only a subset of the tests add the attribute `tests = ["testName"];`


    # Inputs

    `tests`

    : Tests to run

    # Type

    ```
    runTests :: {
      tests = [ String ];
      ${testName} :: {
        expr :: a;
        expected :: a;
      };
    }
    ->
    [
      {
        name :: String;
        expected :: a;
        result :: a;
      }
    ]
    ```

    # Examples
    :::{.example}
    ## `lib.debug.runTests` usage example

    ```nix
    runTests {
      testAndOk = {
        expr = lib.and true false;
        expected = false;
      };
      testAndFail = {
        expr = lib.and true false;
        expected = true;
      };
    }
    ->
    [
      {
        name = "testAndFail";
        expected = true;
        result = false;
      }
    ]
    ```

    :::
  */
  runTests =
    tests: concatLists (attrValues (mapAttrs (name: test:
    let testsToRun = if tests ? tests then tests.tests else [];
    in if (substring 0 4 name == "test" ||  elem name testsToRun)
       && ((testsToRun == []) || elem name tests.tests)
       && (test.expr != test.expected)

      then [ { inherit name; expected = test.expected; result = test.expr; } ]
      else [] ) tests));

  /**
    Create a test assuming that list elements are `true`.


    # Inputs

    `expr`

    : 1\. Function argument


    # Examples
    :::{.example}
    ## `lib.debug.testAllTrue` usage example

    ```nix
    { testX = allTrue [ true ]; }
    ```

    :::
  */
  testAllTrue = expr: { inherit expr; expected = map (x: true) expr; };
}