lib.fileset.trace, lib.fileset.traceVal: init

lib/fileset/README.md

@@ -207,6 +207,5 @@ Here's a list of places in the library that need to be updated in the future:
 - > The file set library is currently somewhat limited but is being expanded to include more functions over time.
 
   in [the manual](../../doc/functions/fileset.section.md)
-- Once a tracing function exists, `__noEval` in [internal.nix](./internal.nix) should mention it
 - If/Once a function to convert `lib.sources` values into file sets exists, the `_coerce` and `toSource` functions should be updated to mention that function in the error when such a value is passed
 - If/Once a function exists that can optionally include a path depending on whether it exists, the error message for the path not existing in `_coerce` should mention the new function

lib/fileset/default.nix

@@ -6,12 +6,14 @@ let
     _coerceMany
     _toSourceFilter
     _unionMany
+    _printFileset
     ;
 
   inherit (builtins)
     isList
     isPath
     pathExists
+    seq
     typeOf
     ;
 
@@ -275,4 +277,93 @@ If a directory does not recursively contain any file, it is omitted from the sto
         _unionMany
       ];
 
+  /*
+    Incrementally evaluate and trace a file set in a pretty way.
+    This function is only intended for debugging purposes.
+    The exact tracing format is unspecified and may change.
+
+    This function takes a final argument to return.
+    In comparison, [`traceVal`](#function-library-lib.fileset.traceVal) returns
+    the given file set argument.
+
+    This variant is useful for tracing file sets in the Nix repl.
+
+    Type:
+      trace :: FileSet -> Any -> Any
+
+    Example:
+      trace (unions [ ./Makefile ./src ./tests/run.sh ]) null
+      =>
+      trace: /home/user/src/myProject
+      trace: - Makefile (regular)
+      trace: - src (all files in directory)
+      trace: - tests
+      trace:   - run.sh (regular)
+      null
+  */
+  trace =
+    /*
+    The file set to trace.
+
+    This argument can also be a path,
+    which gets [implicitly coerced to a file set](#sec-fileset-path-coercion).
+    */
+    fileset:
+    let
+      # "fileset" would be a better name, but that would clash with the argument name,
+      # and we cannot change that because of https://github.com/nix-community/nixdoc/issues/76
+      actualFileset = _coerce "lib.fileset.trace: argument" fileset;
+    in
+    seq
+      (_printFileset actualFileset)
+      (x: x);
+
+  /*
+    Incrementally evaluate and trace a file set in a pretty way.
+    This function is only intended for debugging purposes.
+    The exact tracing format is unspecified and may change.
+
+    This function returns the given file set.
+    In comparison, [`trace`](#function-library-lib.fileset.trace) takes another argument to return.
+
+    This variant is useful for tracing file sets passed as arguments to other functions.
+
+    Type:
+      traceVal :: FileSet -> FileSet
+
+    Example:
+      toSource {
+        root = ./.;
+        fileset = traceVal (unions [
+          ./Makefile
+          ./src
+          ./tests/run.sh
+        ]);
+      }
+      =>
+      trace: /home/user/src/myProject
+      trace: - Makefile (regular)
+      trace: - src (all files in directory)
+      trace: - tests
+      trace:   - run.sh (regular)
+      "/nix/store/...-source"
+  */
+  traceVal =
+    /*
+    The file set to trace and return.
+
+    This argument can also be a path,
+    which gets [implicitly coerced to a file set](#sec-fileset-path-coercion).
+    */
+    fileset:
+    let
+      # "fileset" would be a better name, but that would clash with the argument name,
+      # and we cannot change that because of https://github.com/nix-community/nixdoc/issues/76
+      actualFileset = _coerce "lib.fileset.traceVal: argument" fileset;
+    in
+    seq
+      (_printFileset actualFileset)
+      # We could also return the original fileset argument here,
+      # but that would then duplicate work for consumers of the fileset, because then they have to coerce it again
+      actualFileset;
 }

lib/fileset/internal.nix

@@ -7,11 +7,14 @@ let
     isString
     pathExists
     readDir
-    typeOf
+    seq
     split
+    trace
+    typeOf
     ;
 
   inherit (lib.attrsets)
+    attrNames
     attrValues
     mapAttrs
     setAttrByPath
@@ -103,7 +106,9 @@ rec {
   ];
 
   _noEvalMessage = ''
-    lib.fileset: Directly evaluating a file set is not supported. Use `lib.fileset.toSource` to turn it into a usable source instead.'';
+    lib.fileset: Directly evaluating a file set is not supported.
+      To turn it into a usable source, use `lib.fileset.toSource`.
+      To pretty-print the contents, use `lib.fileset.trace` or `lib.fileset.traceVal`.'';
 
   # The empty file set without a base path
   _emptyWithoutBase = {
@@ -114,8 +119,10 @@ rec {
     # The one and only!
     _internalIsEmptyWithoutBase = true;
 
-    # Double __ to make it be evaluated and ordered first
-    __noEval = throw _noEvalMessage;
+    # Due to alphabetical ordering, this is evaluated last,
+    # which makes the nix repl output nicer than if it would be ordered first.
+    # It also allows evaluating it strictly up to this error, which could be useful
+    _noEval = throw _noEvalMessage;
   };
 
   # Create a fileset, see ./README.md#fileset
@@ -137,8 +144,10 @@ rec {
       _internalBaseComponents = components parts.subpath;
       _internalTree = tree;
 
-      # Double __ to make it be evaluated and ordered first
-      __noEval = throw _noEvalMessage;
+      # Due to alphabetical ordering, this is evaluated last,
+      # which makes the nix repl output nicer than if it would be ordered first.
+      # It also allows evaluating it strictly up to this error, which could be useful
+      _noEval = throw _noEvalMessage;
     };
 
   # Coerce a value to a fileset, erroring when the value cannot be coerced.
@@ -237,22 +246,22 @@ rec {
       // value;
 
   /*
-    Simplify a filesetTree recursively:
-    - Replace all directories that have no files with `null`
+    A normalisation of a filesetTree suitable filtering with `builtins.path`:
+    - Replace all directories that have no files with `null`.
       This removes directories that would be empty
-    - Replace all directories with all files with `"directory"`
+    - Replace all directories with all files with `"directory"`.
       This speeds up the source filter function
 
     Note that this function is strict, it evaluates the entire tree
 
     Type: Path -> filesetTree -> filesetTree
   */
-  _simplifyTree = path: tree:
+  _normaliseTreeFilter = path: tree:
     if tree == "directory" || isAttrs tree then
       let
         entries = _directoryEntries path tree;
-        simpleSubtrees = mapAttrs (name: _simplifyTree (path + "/${name}")) entries;
-        subtreeValues = attrValues simpleSubtrees;
+        normalisedSubtrees = mapAttrs (name: _normaliseTreeFilter (path + "/${name}")) entries;
+        subtreeValues = attrValues normalisedSubtrees;
       in
       # This triggers either when all files in a directory are filtered out
       # Or when the directory doesn't contain any files at all
@@ -262,18 +271,120 @@ rec {
       else if all isString subtreeValues then
         "directory"
       else
-        simpleSubtrees
+        normalisedSubtrees
+    else
+      tree;
+
+  /*
+    A minimal normalisation of a filesetTree, intended for pretty-printing:
+    - If all children of a path are recursively included or empty directories, the path itself is also recursively included
+    - If all children of a path are fully excluded or empty directories, the path itself is an empty directory
+    - Other empty directories are represented with the special "emptyDir" string
+      While these could be replaced with `null`, that would take another mapAttrs
+
+    Note that this function is partially lazy.
+
+    Type: Path -> filesetTree -> filesetTree (with "emptyDir"'s)
+  */
+  _normaliseTreeMinimal = path: tree:
+    if tree == "directory" || isAttrs tree then
+      let
+        entries = _directoryEntries path tree;
+        normalisedSubtrees = mapAttrs (name: _normaliseTreeMinimal (path + "/${name}")) entries;
+        subtreeValues = attrValues normalisedSubtrees;
+      in
+      # If there are no entries, or all entries are empty directories, return "emptyDir".
+      # After this branch we know that there's at least one file
+      if all (value: value == "emptyDir") subtreeValues then
+        "emptyDir"
+
+      # If all subtrees are fully included or empty directories
+      # (both of which are coincidentally represented as strings), return "directory".
+      # This takes advantage of the fact that empty directories can be represented as included directories.
+      # Note that the tree == "directory" check allows avoiding recursion
+      else if tree == "directory" || all (value: isString value) subtreeValues then
+        "directory"
+
+      # If all subtrees are fully excluded or empty directories, return null.
+      # This takes advantage of the fact that empty directories can be represented as excluded directories
+      else if all (value: isNull value || value == "emptyDir") subtreeValues then
+        null
+
+      # Mix of included and excluded entries
+      else
+        normalisedSubtrees
     else
       tree;
 
+  # Trace a filesetTree in a pretty way when the resulting value is evaluated.
+  # This can handle both normal filesetTree's, and ones returned from _normaliseTreeMinimal
+  # Type: Path -> filesetTree (with "emptyDir"'s) -> Null
+  _printMinimalTree = base: tree:
+    let
+      treeSuffix = tree:
+        if isAttrs tree then
+          ""
+        else if tree == "directory" then
+          " (all files in directory)"
+        else
+          # This does "leak" the file type strings of the internal representation,
+          # but this is the main reason these file type strings even are in the representation!
+          # TODO: Consider removing that information from the internal representation for performance.
+          # The file types can still be printed by querying them only during tracing
+          " (${tree})";
+
+      # Only for attribute set trees
+      traceTreeAttrs = prevLine: indent: tree:
+        foldl' (prevLine: name:
+          let
+            subtree = tree.${name};
+
+            # Evaluating this prints the line for this subtree
+            thisLine =
+              trace "${indent}- ${name}${treeSuffix subtree}" prevLine;
+          in
+          if subtree == null || subtree == "emptyDir" then
+            # Don't print anything at all if this subtree is empty
+            prevLine
+          else if isAttrs subtree then
+            # A directory with explicit entries
+            # Do print this node, but also recurse
+            traceTreeAttrs thisLine "${indent}  " subtree
+          else
+            # Either a file, or a recursively included directory
+            # Do print this node but no further recursion needed
+            thisLine
+        ) prevLine (attrNames tree);
+
+      # Evaluating this will print the first line
+      firstLine =
+        if tree == null || tree == "emptyDir" then
+          trace "(empty)" null
+        else
+          trace "${toString base}${treeSuffix tree}" null;
+    in
+    if isAttrs tree then
+      traceTreeAttrs firstLine "" tree
+    else
+      firstLine;
+
+  # Pretty-print a file set in a pretty way when the resulting value is evaluated
+  # Type: fileset -> Null
+  _printFileset = fileset:
+    if fileset._internalIsEmptyWithoutBase then
+      trace "(empty)" null
+    else
+      _printMinimalTree fileset._internalBase
+        (_normaliseTreeMinimal fileset._internalBase fileset._internalTree);
+
   # Turn a fileset into a source filter function suitable for `builtins.path`
   # Only directories recursively containing at least one files are recursed into
   # Type: Path -> fileset -> (String -> String -> Bool)
   _toSourceFilter = fileset:
     let
       # Simplify the tree, necessary to make sure all empty directories are null
       # which has the effect that they aren't included in the result
-      tree = _simplifyTree fileset._internalBase fileset._internalTree;
+      tree = _normaliseTreeFilter fileset._internalBase fileset._internalTree;
 
       # The base path as a string with a single trailing slash
       baseString =