diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index a05dfb734bf5..896c740d8007 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -400,22 +400,18 @@ impl<'a, 'b, 'mir, 'tcx> FnEvalContext<'a, 'b, 'mir, 'tcx> {
 
     fn run(&mut self) -> EvalResult<()> {
         let mut stepper = stepper::Stepper::new(self);
-        'outer: loop {
+        let mut done = false;
+        while !done {
             use self::stepper::Event::*;
-            trace!("// {:?}", stepper.block());
-
-            loop {
-                match stepper.step()? {
-                    Constant => trace!("computing a constant"),
-                    Assignment => trace!("{:?}", stepper.stmt()),
-                    Terminator => {
-                        trace!("{:?}", stepper.term().kind);
-                        continue 'outer;
-                    },
-                    Done => return Ok(()),
-                }
-            }
+            stepper.step(|event| match event {
+                Block(b) => trace!("// {:?}", b),
+                Assignment(a) => trace!("{:?}", a),
+                Terminator(t) => trace!("{:?}", t.kind),
+                Done => done = true,
+                _ => {},
+            })?;
         }
+        Ok(())
     }
 
     fn push_stack_frame(&mut self, def_id: DefId, span: codemap::Span, mir: CachedMir<'mir, 'tcx>, substs: &'tcx Substs<'tcx>,
diff --git a/src/interpreter/stepper.rs b/src/interpreter/stepper.rs
index 7499eb07ae91..d6677c8c319b 100644
--- a/src/interpreter/stepper.rs
+++ b/src/interpreter/stepper.rs
@@ -14,16 +14,18 @@ use syntax::codemap::Span;
 use std::rc::Rc;
 use memory::Pointer;
 
-pub enum Event {
+pub enum Event<'a, 'tcx: 'a> {
+    Block(mir::BasicBlock),
+    Return,
+    Call,
     Constant,
-    Assignment,
-    Terminator,
+    Assignment(&'a mir::Statement<'tcx>),
+    Terminator(&'a mir::Terminator<'tcx>),
     Done,
 }
 
 pub struct Stepper<'fncx, 'a: 'fncx, 'b: 'a + 'mir, 'mir: 'fncx, 'tcx: 'b>{
     fncx: &'fncx mut FnEvalContext<'a, 'b, 'mir, 'tcx>,
-    process: fn (&mut Stepper<'fncx, 'a, 'b, 'mir, 'tcx>) -> EvalResult<()>,
 
     // a cache of the constants to be computed before the next statement/terminator
     // this is an optimization, so we don't have to allocate a new vector for every statement
@@ -34,17 +36,15 @@ impl<'fncx, 'a, 'b: 'a + 'mir, 'mir, 'tcx: 'b> Stepper<'fncx, 'a, 'b, 'mir, 'tcx
     pub(super) fn new(fncx: &'fncx mut FnEvalContext<'a, 'b, 'mir, 'tcx>) -> Self {
         Stepper {
             fncx: fncx,
-            process: Self::dummy,
             constants: Vec::new(),
         }
     }
 
-    fn dummy(&mut self) -> EvalResult<()> { Ok(()) }
-
-    fn statement(&mut self) -> EvalResult<()> {
+    fn statement<F: for<'f> FnMut(Event<'f, 'tcx>)>(&mut self, mut f: F) -> EvalResult<()> {
         let mir = self.fncx.mir();
         let block_data = mir.basic_block_data(self.fncx.frame().next_block);
         let stmt = &block_data.statements[self.fncx.frame().stmt];
+        f(Event::Assignment(stmt));
         let mir::StatementKind::Assign(ref lvalue, ref rvalue) = stmt.kind;
         let result = self.fncx.eval_assignment(lvalue, rvalue);
         self.fncx.maybe_report(result)?;
@@ -52,33 +52,33 @@ impl<'fncx, 'a, 'b: 'a + 'mir, 'mir, 'tcx: 'b> Stepper<'fncx, 'a, 'b, 'mir, 'tcx
         Ok(())
     }
 
-    fn terminator(&mut self) -> EvalResult<()> {
+    fn terminator<F: for<'f> FnMut(Event<'f, 'tcx>)>(&mut self, mut f: F) -> EvalResult<()> {
         // after a terminator we go to a new block
         self.fncx.frame_mut().stmt = 0;
         let term = {
             let mir = self.fncx.mir();
             let block_data = mir.basic_block_data(self.fncx.frame().next_block);
             let terminator = block_data.terminator();
+            f(Event::Terminator(terminator));
             let result = self.fncx.eval_terminator(terminator);
             self.fncx.maybe_report(result)?
         };
         match term {
-            TerminatorTarget::Block => {},
+            TerminatorTarget::Block => f(Event::Block(self.fncx.frame().next_block)),
             TerminatorTarget::Return => {
+                f(Event::Return);
                 self.fncx.pop_stack_frame();
             },
-            TerminatorTarget::Call => {},
+            TerminatorTarget::Call => f(Event::Call),
         }
         Ok(())
     }
 
-    pub fn step(&mut self) -> EvalResult<Event> {
-        (self.process)(self)?;
-
+    // returns true as long as there are more things to do
+    pub fn step<F: for<'f> FnMut(Event<'f, 'tcx>)>(&mut self, mut f: F) -> EvalResult<()> {
         if self.fncx.stack.is_empty() {
-            // fuse the iterator
-            self.process = Self::dummy;
-            return Ok(Event::Done);
+            f(Event::Done);
+            return Ok(());
         }
 
         let block = self.fncx.frame().next_block;
@@ -97,12 +97,9 @@ impl<'fncx, 'a, 'b: 'a + 'mir, 'mir, 'tcx: 'b> Stepper<'fncx, 'a, 'b, 'mir, 'tcx
                 mir: &mir,
             }.visit_statement(block, stmt);
             if self.constants.is_empty() {
-                self.process = Self::statement;
-                return Ok(Event::Assignment);
+                return self.statement(f);
             } else {
-                self.process = Self::statement;
-                self.extract_constants();
-                return Ok(Event::Constant);
+                return self.extract_constants(f);
             }
         }
 
@@ -117,36 +114,21 @@ impl<'fncx, 'a, 'b: 'a + 'mir, 'mir, 'tcx: 'b> Stepper<'fncx, 'a, 'b, 'mir, 'tcx
             mir: &mir,
         }.visit_terminator(block, terminator);
         if self.constants.is_empty() {
-            self.process = Self::terminator;
-            Ok(Event::Terminator)
+            self.terminator(f)
         } else {
-            self.process = Self::statement;
-            self.extract_constants();
-            Ok(Event::Constant)
+            self.extract_constants(f)
         }
     }
 
-    fn extract_constants(&mut self) {
+    fn extract_constants<F: for<'f> FnMut(Event<'f, 'tcx>)>(&mut self, mut f: F) -> EvalResult<()> {
         assert!(!self.constants.is_empty());
         for (cid, span, return_ptr, mir) in self.constants.drain(..) {
             let def_id = cid.def_id();
             let substs = cid.substs();
+            f(Event::Constant);
             self.fncx.push_stack_frame(def_id, span, mir, substs, Some(return_ptr));
         }
-    }
-
-    /// returns the statement that will be processed next
-    pub fn stmt(&self) -> &mir::Statement {
-        &self.fncx.basic_block().statements[self.fncx.frame().stmt]
-    }
-
-    /// returns the terminator of the current block
-    pub fn term(&self) -> &mir::Terminator {
-        self.fncx.basic_block().terminator()
-    }
-
-    pub fn block(&self) -> mir::BasicBlock {
-        self.fncx.frame().next_block
+        self.step(f)
     }
 }