diff --git a/TrainingData/Frontend.lean b/TrainingData/Frontend.lean
index c687eb5..0d4baa8 100644
--- a/TrainingData/Frontend.lean
+++ b/TrainingData/Frontend.lean
@@ -81,6 +81,8 @@ the `src : Substring` and `stx : Syntax` of the command,
 and any `Message`s and `InfoTree`s produced while processing.
 -/
 structure CompilationStep where
+  fileName : String
+  fileMap : FileMap
   src : Substring
   stx : Syntax
   before : Environment
@@ -104,7 +106,8 @@ def one : FrontendM (CompilationStep × Bool) := do
   let after := s'.env
   let msgs := s'.messages.msgs.drop s.messages.msgs.size
   let trees := s'.infoState.trees.drop s.infoState.trees.size
-  return ({ src, stx, before, after, msgs, trees }, done)
+  let ⟨_, fileName, fileMap⟩  := (← read).inputCtx
+  return ({ fileName, fileMap, src, stx, before, after, msgs, trees }, done)
 
 /-- Process all commands in the input. -/
 partial def all : FrontendM (List CompilationStep) := do
@@ -114,11 +117,35 @@ partial def all : FrontendM (List CompilationStep) := do
   else
     return cmd :: (← all)
 
+def runCoreMBefore (c : CompilationStep) (x : CoreM α) : IO α :=
+  (·.1) <$> Core.CoreM.toIO x { fileName := c.fileName, fileMap := c.fileMap } { env := c.before }
+
+open Meta in
+def runMetaMBefore (c : CompilationStep) (x : MetaM α) : IO α :=
+  c.runCoreMBefore <| MetaM.run' x {} {}
+
 /-- Return all new `ConstantInfo`s added during the processed command. -/
 def diff (cmd : CompilationStep) : List ConstantInfo :=
   cmd.after.constants.map₂.toList.filterMap
     fun (c, i) => if cmd.before.constants.map₂.contains c then none else some i
 
+/-- Data extracted from a `ConstantInfo`. -/
+structure DeclInfo where
+  name : Name
+  type : Expr
+  ppType : String
+  docString : Option String
+
+/-- Return info about each new declaration added during the processed command. -/
+def newDecls (cmd : CompilationStep) : IO (List DeclInfo) := do
+  cmd.diff.mapM fun ci =>
+    return {
+      name := ci.name
+      type := ci.type
+      ppType := toString (← cmd.runMetaMBefore <| Meta.ppExpr ci.type)
+      docString := ← findDocString? cmd.after ci.name
+    }
+
 end CompilationStep
 
 /--
@@ -188,7 +215,7 @@ open System
 
 -- TODO allow finding Lean 4 sources from the toolchain.
 def findLean (mod : Name) : IO FilePath := do
-  return FilePath.mk ((← findOLean mod).toString.replace "build/lib/" "") |>.withExtension "lean"
+  return FilePath.mk ((← findOLean mod).toString.replace ".lake/build/lib/" "") |>.withExtension "lean"
 
 /-- Implementation of `moduleSource`, which is the cached version of this function. -/
 def moduleSource' (mod : Name) : IO String := do
diff --git a/lakefile.lean b/lakefile.lean
index 0de323c..dacd289 100644
--- a/lakefile.lean
+++ b/lakefile.lean
@@ -29,6 +29,11 @@ lean_exe training_data where
   root := `scripts.training_data
   supportInterpreter := true
 
+@[default_target]
+lean_exe comment_data where
+  root := `scripts.comment_data
+  supportInterpreter := true
+
 @[default_target]
 lean_exe premises where
   root := `scripts.premises
diff --git a/scripts/comment_data.lean b/scripts/comment_data.lean
new file mode 100644
index 0000000..1083089
--- /dev/null
+++ b/scripts/comment_data.lean
@@ -0,0 +1,64 @@
+import TrainingData.Frontend
+import TrainingData.InfoTree.ToJson
+import TrainingData.InfoTree.TacticInvocation.Basic
+import Mathlib.Data.String.Defs
+import Mathlib.Lean.CoreM
+import Std.Lean.Util.Path
+import Std.Data.String.Basic
+import Cli
+
+open Lean Elab IO Meta
+open Cli System
+
+structure CommentData where
+  declName : String
+  declType : String
+  docString : Option String
+  context : String
+  decl : String
+deriving ToJson, FromJson
+
+def addStep (data : String × List CommentData) (step : CompilationStep) :
+    IO (String × List CommentData) := do
+  let (context, prev) := data
+  let decl := step.src.toString
+  let context := context ++ decl
+  let new ← step.newDecls
+  let next := new.map fun d =>
+  { context
+    docString := d.docString
+    declName := d.name.toString
+    declType := d.ppType
+    decl }
+  return (context, next ++ prev)
+
+def commentData (args : Cli.Parsed) : IO UInt32 := do
+    searchPathRef.set compile_time_search_path%
+    let module := args.positionalArg! "module" |>.as! ModuleName
+    let steps ← compileModule module
+    let (_, data) ← steps.foldlM (init := ("", [])) addStep
+    IO.println <| toJson data
+    return 0
+
+/-- Setting up command line options and help text for `lake exe comment_data`. -/
+def comment_data : Cmd := `[Cli|
+  comment_data VIA commentData; ["0.0.1"]
+"Export doc-string training data from the given file.
+
+The output is a json array of dictionaries with fields
+* `declName`: the declaration name
+* `declType`: the pretty-printed type of the declaration
+* `docString`: the declaration doc-string, if it is present
+* `decl`: the entire body of the declaration
+* `context`: the file source up to before the declaration
+"
+
+  ARGS:
+    module : ModuleName; "Lean module to compile and export training data."
+]
+
+/-- `lake exe comment_data` -/
+def main (args : List String) : IO UInt32 :=
+  comment_data.validate args
+
+-- See `scripts/comment_data.sh` for a script to run this on all of Mathlib.
diff --git a/scripts/comment_data.sh b/scripts/comment_data.sh
new file mode 100755
index 0000000..1171ef7
--- /dev/null
+++ b/scripts/comment_data.sh
@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+
+## Extracts doc-strings for every declaration.
+
+# See the help text in `lake exe comment_data` for a description of the output format.
+
+# Run either as `scripts/comment_data.sh` to run on all of Mathlib (several hours),
+# or `scripts/comment_data.sh Mathlib.Logic.Hydra` to run on just one file.
+# Results will go in `out/comment_data/Mathlib.Logic.Hydra.comments`.
+
+FLAGS=()
+ARGS=()
+
+for arg in "$@"; do
+    if [[ $arg == --* ]]; then
+        FLAGS+=("$arg")
+    else
+        ARGS+=("$arg")
+    fi
+done
+
+if [ ${#ARGS[@]} -eq 0 ]; then
+  lake build comment_data
+  parallel -j32 ./scripts/comment_data.sh ${FLAGS[@]} -- ::: `cat .lake/packages/mathlib/Mathlib.lean | sed -e 's/import //'`
+else
+  DIR=out/comment_data
+  mkdir -p $DIR
+  mod=${ARGS[0]}
+  if [ ! -f $DIR/$mod.comment ]; then
+    echo $mod
+    if [ ! -f .lake/build/bin/comment_data ]; then
+      lake build comment_data
+    fi
+    .lake/build/bin/comment_data ${FLAGS[@]} $mod > $DIR/$mod._comment && mv $DIR/$mod._comment $DIR/$mod.comment
+  fi
+fi