diff --git a/library/init/category/reader.lean b/library/init/category/reader.lean
index d40a16c115..b13b12910c 100644
--- a/library/init/category/reader.lean
+++ b/library/init/category/reader.lean
@@ -117,6 +117,19 @@ instance [monad m] : monad_reader_adapter ρ ρ' (reader_t ρ m) (reader_t ρ' m
 ⟨λ α, reader_t.adapt⟩
 end
 
-
 instance (ρ : Type u) (m out) [monad_run out m] : monad_run (λ α, ρ → out α) (reader_t ρ m) :=
 ⟨λ α x, run ∘ x.run⟩
+
+class monad_reader_runner (ρ : Type u) (m m' : Type u → Type u) :=
+(run_reader {} {α : Type u} : m α → ρ → m' α)
+export monad_reader_runner (run_reader)
+
+section
+variables {ρ ρ' : Type u} {m m' : Type u → Type u}
+
+instance monad_reader_runner_trans {n n' : Type u → Type u} [monad_functor m m' n n'] [monad_reader_runner ρ m m'] : monad_reader_runner ρ n n' :=
+⟨λ α x r, monad_map (λ α (y : m α), (run_reader y r : m' α)) x⟩
+
+instance reader_t.monad_state_runner [monad m] : monad_reader_runner ρ (reader_t ρ m) m :=
+⟨λ α x r, x.run r⟩
+end
diff --git a/library/init/category/state.lean b/library/init/category/state.lean
index 916de397ff..ce8d244904 100644
--- a/library/init/category/state.lean
+++ b/library/init/category/state.lean
@@ -170,6 +170,19 @@ instance [monad m] : monad_state_adapter σ σ' (state_t σ m) (state_t σ' m) :
 ⟨λ σ'' α, state_t.adapt⟩
 end
 
-
 instance (σ m out) [monad_run out m] : monad_run (λ α, σ → out (α × σ)) (state_t σ m) :=
 ⟨λ α x, run ∘ (λ σ, x.run σ)⟩
+
+class monad_state_runner (σ : Type u) (m m' : Type u → Type u) :=
+(run_state {} {α : Type u} : m α → σ → m' α)
+export monad_state_runner (run_state)
+
+section
+variables {σ σ' : Type u} {m m' : Type u → Type u}
+
+instance monad_state_runner_trans {n n' : Type u → Type u} [monad_functor m m' n n'] [monad_state_runner σ m m'] : monad_state_runner σ n n' :=
+⟨λ α x s, monad_map (λ α (y : m α), (run_state y s : m' α)) x⟩
+
+instance state_t.monad_state_runner [monad m] : monad_state_runner σ (state_t σ m) m :=
+⟨λ α x s, prod.fst <$> x.run s⟩
+end
diff --git a/library/init/compiler/ir.lean b/library/init/compiler/ir.lean
index 9abbd943c4..250313d92e 100644
--- a/library/init/compiler/ir.lean
+++ b/library/init/compiler/ir.lean
@@ -47,10 +47,10 @@ show has_lt string, from infer_instance
 def var_set        := rbtree var (<)
 def blockid_set    := rbtree blockid (<)
 def context        := rbmap var type (<)
-def mk_var_set     := mk_rbtree var (<)
-def mk_blockid_set := mk_rbtree blockid (<)
 def var2blockid    := rbmap var blockid (<)
-def mk_var2blockid := mk_rbmap var blockid (<)
+def mk_var_set     : var_set     := mk_rbtree var (<)
+def mk_blockid_set : blockid_set := mk_rbtree blockid (<)
+def mk_var2blockid : var2blockid := mk_rbmap var blockid (<)
 
 inductive instr
 | lit     (x : var) (ty : type) (lit : literal)              -- x : ty := lit
@@ -107,12 +107,19 @@ SSA validator
 @[reducible] def ssa_check : Type :=
 except_t string (state (var2blockid × var_set)) unit
 
-def var.declare_at (b : blockid) (x : var) : ssa_check :=
-do (m, s) ← get,
-   if m.contains x then throw ("variable has already been defined '" ++ x ++ "'")
-   else put (m.insert x b, s)
+inductive ssa_error
+| already_defined (v : var)
+| undefined (v : var)
+| no_block
 
-def instr.declare_vars_at (b : blockid) : instr → ssa_check
+@[reducible] def ssa_decl_m := except_t ssa_error (state_t var2blockid id)
+
+def var.declare_at (b : blockid) (x : var) : ssa_decl_m unit :=
+do m ← get,
+   if m.contains x then throw $ ssa_error.already_defined x
+   else put (m.insert x b)
+
+def instr.declare_vars_at (b : blockid) : instr → ssa_decl_m unit
 | (instr.lit x _ _)       := x.declare_at b
 | (instr.cast x _ _)      := x.declare_at b
 | (instr.unop x _ _ _)    := x.declare_at b
@@ -131,57 +138,59 @@ def instr.declare_vars_at (b : blockid) : instr → ssa_check
 | (instr.sread x _ _ _)   := x.declare_at b
 | _                       := return ()
 
-def phi.declare_at (b : blockid) : phi → ssa_check
+def phi.declare_at (b : blockid) : phi → ssa_decl_m unit
 | {x := x, ..} := x.declare_at b
 
-def block.declare_vars : block → ssa_check
+def block.declare_vars : block → ssa_decl_m unit
 | {id := b, phis := ps, instrs := is, ..} :=
   ps.mmap' (phi.declare_at b) >>
   is.mmap' (instr.declare_vars_at b)
 
-def arg.declare_at (b : blockid) : arg → ssa_check
+def arg.declare_at (b : blockid) : arg → ssa_decl_m unit
 | {n := x, ..} := x.declare_at b
 
 /- Collect where each variable is declared, and
    check whether each variable was declared at most once. -/
-def decl.declare_vars : decl → ssa_check
+def decl.declare_vars : decl → ssa_decl_m unit
 | {as := as, bs := b::bs, ..} :=
   /- We assume that arguments are declared in the first basic block.
      TODO: check whether this assumption matches LLVM or not -/
   as.mmap' (arg.declare_at b.id) >>
   b.declare_vars >>
   bs.mmap' block.declare_vars
-| _ := throw "declaration must have at least one block"
+| _ := throw ssa_error.no_block
 
 /- Generate the mapping from variable to blockid for the given declaration.
    This function assumes `d` is in SSA. -/
-def decl.var2blockid (d : decl) : var2blockid :=
-let (_, (m, _)) := d.declare_vars.run.run (mk_var2blockid, mk_var_set) in
-m
+def decl.var2blockid (d : decl) : except_t ssa_error id var2blockid :=
+run_state (d.declare_vars >> get) mk_var2blockid
+
+@[reducible] def ssa_valid_m := except_t ssa_error (reader_t var2blockid (state_t var_set id))
 
 /- Given, x := phi ys,
    check whether every ys is declared at the var2blockid mapping,
    and update the set of already defined variables in the basic block with `x`.
 
    TODO: check whether the SSA validation rules here match the ones used in LLVM. -/
-def phi.valid_ssa : phi → ssa_check
+def phi.valid_ssa : phi → ssa_valid_m unit
 | {x := x, ys := ys, ..} := do
-  (m, s) ← get,
+  m ← read,
   ys.mmap' (λ y, if m.contains y then return ()
-                 else throw ("undefined '" ++ y ++ "'")),
-  put (m, s.insert x)
+                 else throw $ ssa_error.undefined y),
+  s ← get,
+  put (s.insert x)
 
 /- Check whether `x` has been already defined in the current basic block or not. -/
-def var.defined (x : var) : ssa_check :=
-do (_, s) ← get,
+def var.defined (x : var) : ssa_valid_m unit :=
+do s ← get,
    if s.contains x then return ()
-   else throw ("undefined variable '" ++ x ++ "'")
+   else throw $ ssa_error.undefined x
 
 /- Mark `x` as a variable defined in the current basic block. -/
-def var.define (x : var) : ssa_check :=
-do (m, s) ← get, put (m, s.insert x)
+def var.define (x : var) : ssa_valid_m unit :=
+do s ← get, put (s.insert x)
 
-def instr.valid_ssa : instr → ssa_check
+def instr.valid_ssa : instr → ssa_valid_m unit
 | (instr.lit x _ _)       := x.define
 | (instr.cast x _ y)      := x.define >> y.defined
 | (instr.unop x _ _ y)    := x.define >> y.defined
@@ -205,63 +214,65 @@ def instr.valid_ssa : instr → ssa_check
 | (instr.dealloc x)       := x.defined
 | (instr.dec x)           := x.defined
 
-def terminator.valid_ssa : terminator → ssa_check
+def terminator.valid_ssa : terminator → ssa_valid_m unit
 | (terminator.ret ys)     := ys.mmap' var.defined
 | (terminator.case x _)   := x.defined
 | (terminator.jmp _)      := return ()
 
-def block.valid_ssa : block → ssa_check
+def block.valid_ssa_core : block → ssa_valid_m unit
 | {phis := ps, instrs := is, term := r, ..} :=
-  do (m, s) ← get,
-     put (m, mk_var_set), -- reset set of variables defined in the current block
-     ps.mmap' phi.valid_ssa,
+  do ps.mmap' phi.valid_ssa,
      is.mmap' instr.valid_ssa,
      r.valid_ssa
 
+def block.valid_ssa (b : block) : except_t ssa_error (reader_t var2blockid id) unit :=
+run_state b.valid_ssa_core mk_var_set
+
 /-
 We first check whether every variable `x` was declared only once
 and store the blockid where `x` is defined (action: `decl.declare_vars`).
 Then, we check whether every used variable in basic block has been
 defined before being used.
 -/
-def decl.valid_ssa : decl → ssa_check
+def decl.valid_ssa : decl → except_t ssa_error id var2blockid
 | d@{as := as, bs := bs, ..} :=
-  d.declare_vars >> bs.mmap' block.valid_ssa
+  do m ← d.var2blockid,
+     bs.mmap' (λ b : block, run_reader b.valid_ssa m),
+     return m
 
 def valid_ssa (d : decl) : bool :=
-let (e, _) := d.valid_ssa.run.run (mk_var2blockid, mk_var_set)
-in  e.to_bool
+d.valid_ssa.run.to_bool
 
 /- Check blockids -/
-@[reducible] def blockid_check : Type :=
-except_t string (state blockid_set) unit
+@[reducible] def blockid_check_m :=
+except_t string (state blockid_set)
 
-def block.declare : block → blockid_check
+def block.declare : block → blockid_check_m unit
 | {id := id, ..} :=
   do s ← get,
      if s.contains id then throw ("blockid '" ++ id ++ "' has already been used")
      else put (s.insert id)
 
-def blockid.defined (bid : blockid) : blockid_check :=
+def blockid.defined (bid : blockid) : blockid_check_m unit :=
 do s ← get,
    if s.contains bid then return ()
    else throw ("unknown blockid '" ++ bid ++ "'")
 
-def terminator.check_blockids : terminator → blockid_check
+def terminator.check_blockids : terminator → blockid_check_m unit
 | (terminator.ret ys)      := return ()
 | (terminator.case _ bids) := bids.mmap' blockid.defined
 | (terminator.jmp bid)     := bid.defined
 
-def block.check_blockids : block → blockid_check
+def block.check_blockids : block → blockid_check_m unit
 | {term := r, ..} := r.check_blockids
 
-def decl.check_blockids : decl → blockid_check
+def decl.check_blockids : decl → blockid_check_m unit
 | {bs := bs, ..} :=
   bs.mmap' block.declare >> bs.mmap' block.check_blockids
 
 def check_blockids (d : decl) : bool :=
-let (e, _) := d.check_blockids.run.run mk_blockid_set
-in  e.to_bool
+let r : except_t string id unit := run_state d.check_blockids mk_blockid_set in
+r.run.to_bool
 
 /-
 TODO: type inference