refactor: output print-paths build info to stderr

Lake/BuildBin.lean

@@ -32,7 +32,7 @@ def Package.staticLibTarget (self : Package) : FileTarget :=
     (← self.buildTarget).staticLibTarget.materializeAsync
 
 def buildLib (pkg : Package) : IO PUnit :=
-  pkg.staticLibTarget.build.run
+  pkg.staticLibTarget.build.run'
 
 -- # Build Package Bin
 
@@ -52,4 +52,4 @@ def Package.binTarget (self : Package) : FileTarget :=
     pkgTarget.binTarget depTargets >>= (·.materializeAsync)
 
 def buildBin (pkg : Package) : IO PUnit :=
-  pkg.binTarget.build.run
+  pkg.binTarget.build.run'

Lake/BuildMonad.lean

@@ -62,6 +62,12 @@ def io : BuildContext where
     logError := fun msg => IO.eprintln msg
   }
 
+def ioErr : BuildContext where
+  methodsRef := BuildMethodsRef.mk {
+    logInfo  := fun msg => IO.eprintln msg
+    logError := fun msg => IO.eprintln msg
+  }
+
 def methods (self : BuildContext) : BuildMethods :=
   self.methodsRef.get
 
@@ -78,11 +84,8 @@ def logInfo (msg : String) : BuildM PUnit := do
 def logError (msg : String) : BuildM PUnit := do
   (← getMethods).logError msg
 
-def runIn (ctx : BuildContext) (self : BuildM α) : IO α :=
-  self ctx
-
-def run' (self : BuildM α) : IO α :=
-  runIn BuildContext.io do try self catch e =>
+def convErrors (self : BuildM α) : BuildM α := do
+  try self catch e =>
     /-
       Remark: Actual error should have already been logged earlier.
       However, if the error was thrown by something that did not know it was
@@ -93,8 +96,17 @@ def run' (self : BuildM α) : IO α :=
     BuildM.logError s!"build error: {toString e}"
     throw <| IO.userError "build failed"
 
-def run (self : BuildM α) : IO PUnit :=
-  discard self.run'
+def runIn (ctx : BuildContext) (self : BuildM α) : IO α :=
+  self ctx
+
+def run (self : BuildM α) : IO α :=
+  (convErrors self).runIn BuildContext.io
+
+def runErr (self : BuildM α) : IO α :=
+  (convErrors self).runIn BuildContext.ioErr
+
+def run' (self : BuildM α) : IO PUnit :=
+  discard self.run
 
 end BuildM
 

Lake/BuildPackage.lean

@@ -146,7 +146,7 @@ def Package.buildDeps (self : Package) : BuildM (Array Package) := do
   targets.mapM fun target => Functor.mapConst target.info.1 target.materialize
 
 def configure (pkg : Package) : IO Unit :=
-  pkg.buildDeps.run
+  pkg.buildDeps.run'
 
 def Package.build (self : Package) : BuildM PUnit := do
   let depTargets ← self.buildDepTargets
@@ -170,7 +170,7 @@ def Package.filterLocalImports (imports : List String) (self : Package) : Array
   return localImports
 
 def printPaths (pkg : Package) (imports : List String := []) : IO Unit := do
-  let deps ← BuildM.run' do
+  let deps ← BuildM.runErr do
     -- resolve and build deps
     let depTargets ← pkg.buildDepTargets
     let depPkgs := depTargets.map (·.package) |>.foldl (flip List.cons) []