diff --git a/languages/source.nim b/languages/source.nim
index 7ad2b8ba..0837fdb6 100644
--- a/languages/source.nim
+++ b/languages/source.nim
@@ -29,6 +29,7 @@ const lang* = language:
     Call(+expr)
     FieldAccess(expr, IntVal(z))
     At(expr, expr)
+    As(expr, texpr)
     And(expr, expr)
     Or(expr, expr)
     If(expr, expr, expr)
@@ -124,6 +125,8 @@ const lang* = language:
       FieldAccess(xfrm(e_1), IntVal(z_1))
     of At(e_1, e_2):
       At(xfrm(e_1), xfrm(e_2))
+    of As(e_1, texpr_1):
+      As(xfrm(e_1), texpr_1)
     of While(e_1, e_2):
       While(xfrm(e_1), xfrm(e_2))
     of Return(e_1):
@@ -350,6 +353,13 @@ const lang* = language:
       where SeqTy(typ_2), typ_1
       conclusion C_1, At(e_1, e_2), mut(typ_2)
 
+    rule "S-as":
+      premise mtypes(C_1, e_1, typ_1)
+      premise ttypes(C_1, texpr_1, typ_2)
+      condition typ_2 != VoidTy()
+      condition typ_1 <:= typ_2
+      conclusion C_1, As(e_1, texpr_1), typ_2
+
     rule "S-asgn":
       premise types(C_1, e_1, mut(typ_1))
       premise mtypes(C_1, e_2, typ_2)
@@ -575,6 +585,8 @@ const lang* = language:
       FieldAccess(substitute(e_1, with), IntVal(z_1))
     of At(e_1, e_2):
       At(substitute(e_1, with), substitute(e_2, with))
+    of As(e_1, texpr_1):
+      As(substitute(e_1, with), texpr_1)
     of While(e_1, e_2):
       While(substitute(e_1, with), substitute(e_2, with))
     of Return(e_1):
@@ -696,6 +708,7 @@ const lang* = language:
     At(E, e)
     At(le, E)
     At(val, E)
+    As(E, texpr)
     Asgn(E, e)
     Asgn(le, E)
     With(E, n, e)
@@ -715,6 +728,7 @@ const lang* = language:
     Exprs(hole, +e)
     At(le, hole)
     At(val, hole)
+    As(hole, texpr)
     Asgn(le, hole)
     With(hole, n, e)
     With(val, n, hole)
@@ -744,6 +758,7 @@ const lang* = language:
     axiom "E-exprs", Exprs(TupleCons(), +e_1), Exprs(...e_1)
     axiom "E-if-true", If(True, e_1, e_2), e_1
     axiom "E-if-false", If(False, e_1, e_2), e_2
+    axiom "E-as", As(val_1, texpr), val_1 # a no-op
 
     axiom "E-while", While(e_1, e_2), If(e_1, Exprs(e_2, While(e_1, e_2)), TupleCons())
 
diff --git a/languages/specification.md b/languages/specification.md
index eab11f18..8c525560 100644
--- a/languages/specification.md
+++ b/languages/specification.md
@@ -20,6 +20,7 @@ expr     ::= <ident>
           |  (Call <expr>+)
           |  (FieldAccess <expr> <intVal>)
           |  (At <expr> <expr>)
+          |  (As <expr> <texpr>)
           |  (And <expr> <expr>)
           |  (Or <expr> <expr>)
           |  (If <expr> <expr> <expr>?)
diff --git a/passes/source2il.nim b/passes/source2il.nim
index 491e5413..6dd39bc6 100644
--- a/passes/source2il.nim
+++ b/passes/source2il.nim
@@ -697,7 +697,11 @@ proc fitExpr(c; e: sink Expr, target: SemType): Expr =
   else:
     # TODO: this needs a better error message
     c.error("type mismatch")
-    result = Expr(stmts: e.stmts, typ: errorType())
+    # still return a proper expression so that analysis can carry on
+    if target.kind == tkVoid:
+      result = Expr(typ: target, stmts: e.stmts)
+    else:
+      result = Expr(typ: target, stmts: e.stmts, expr: e.expr)
 
 proc fitExprStrict(c; e: sink Expr, typ: SemType): Expr =
   ## Makes sure expression `e` fits `typ` exactly, reporting an error and
@@ -1274,6 +1278,17 @@ proc exprToIL(c; t: InTree, n: NodeIndex, expr, stmts): ExprType =
         newCall(at, @[c.inlineLvalue(arr, stmts), c.capture(idx, stmts)]))
     else:
       result = errorType() + arr.attribs
+  of SourceKind.As:
+    let
+      (a, b) = t.pair(n)
+      e = c.exprToIL(t, a)
+      typ = c.expectNot(c.evalType(t, b), tkVoid)
+    # a copy must always be created, even when there's no widening going on
+    if e.typ == typ:
+      expr = use(c, e, stmts)
+    else:
+      expr = inline(c.fitExpr(e, typ), stmts)
+    result = typ + {} # lvalue-ness and mutability are discarded
   of SourceKind.Asgn:
     let (a, b) = t.pair(n)
     var dst = c.exprToIL(t, a)
diff --git a/passes/syntax_source.nim b/passes/syntax_source.nim
index 84abe987..109cd66d 100644
--- a/passes/syntax_source.nim
+++ b/passes/syntax_source.nim
@@ -20,6 +20,7 @@ type
     TupleCons
     Seq
     FieldAccess, At
+    As
     Exprs
     Asgn
     Return
@@ -34,7 +35,8 @@ type
 
 const
   ExprNodes* = {IntVal, FloatVal, Ident, And, Or, If, While, Call, TupleCons,
-                Seq, FieldAccess, At, Asgn, Return, Unreachable, Exprs, Decl}
+                Seq, FieldAccess, At, As, Asgn, Return, Unreachable, Exprs,
+                Decl}
   DeclNodes* = {ProcDecl, TypeDecl}
   AllNodes* = {low(NodeKind) .. high(NodeKind)}
 
diff --git a/tests/expr/spectest.nim b/tests/expr/spectest.nim
index 8fcdd233..53b55748 100644
--- a/tests/expr/spectest.nim
+++ b/tests/expr/spectest.nim
@@ -57,9 +57,6 @@ const
     "t05_proc_with_union_return_type.test",
     "t05_return_operand_cannot_be_void.test",
     "t05_return_type_mismatch.test",
-    "t05_union_type_1.test",
-    "t05_union_type_2.test",
-    "t05_union_type_of_single_type.test",
     "t05_unreachable_is_void.test",
     "t06_call_lookup_error.test",
     "t06_call_lookup_self_visible.test",
@@ -75,10 +72,6 @@ const
     "t06_redeclaration_error_1.test",
     "t06_redeclaration_error_2.test",
     "t06_redeclaration_error_3.test",
-    "t06_union_duplicate_types.test",
-    "t08_if_expr_unify_type_1.test",
-    "t08_if_expr_unify_type_2.test",
-    "t09_asgn_compatible_type.test",
     "t09_asgn_field.test",
     "t09_decl_void_error.test",
     "t10_asgn_lhs_must_be_lvalue_8.test",
@@ -116,7 +109,6 @@ const
     "t17_seq_copy_2.test",
     "t17_seq_copy_3.test",
     "t17_seq_copy_4.test",
-    "t17_seq_nested_copy_3.test",
     "t18_seq_character_string_1.test",
     "t18_seq_character_string_2.test",
     "t19_write.test",
diff --git a/tests/expr/t02_as.test b/tests/expr/t02_as.test
new file mode 100644
index 00000000..c90e4b68
--- /dev/null
+++ b/tests/expr/t02_as.test
@@ -0,0 +1,4 @@
+discard """
+  output: "1 : (IntTy)"
+"""
+(As 1 (IntTy))
diff --git a/tests/expr/t02_as_no_void_error.test b/tests/expr/t02_as_no_void_error.test
new file mode 100644
index 00000000..8eb586fe
--- /dev/null
+++ b/tests/expr/t02_as_no_void_error.test
@@ -0,0 +1,4 @@
+discard """
+  reject: true
+"""
+(As 1 (VoidTy))
diff --git a/tests/expr/t02_as_type_mismatch_error.test b/tests/expr/t02_as_type_mismatch_error.test
new file mode 100644
index 00000000..9c60978a
--- /dev/null
+++ b/tests/expr/t02_as_type_mismatch_error.test
@@ -0,0 +1,4 @@
+discard """
+  reject: true
+"""
+(As 1 (UnitTy))
diff --git a/tests/expr/t05_union_type_1.test b/tests/expr/t05_union_type_1.test
index 627d523d..1319b0d4 100644
--- a/tests/expr/t05_union_type_1.test
+++ b/tests/expr/t05_union_type_1.test
@@ -1,5 +1,4 @@
 discard """
   output: "100 : (UnionTy (IntTy) (FloatTy))"
 """
-(ProcDecl (Ident "a") (UnionTy (IntTy) (FloatTy)) (Params)
-  (Return (IntVal 100)))
+(As 100 (UnionTy (IntTy) (FloatTy)))
diff --git a/tests/expr/t05_union_type_2.test b/tests/expr/t05_union_type_2.test
index 2090c2d9..65b02654 100644
--- a/tests/expr/t05_union_type_2.test
+++ b/tests/expr/t05_union_type_2.test
@@ -1,5 +1,4 @@
 discard """
   output: "1.5 : (UnionTy (IntTy) (FloatTy))"
 """
-(ProcDecl (Ident "a") (UnionTy (IntTy) (FloatTy)) (Params)
-  (Return (FloatVal 1.5)))
+(As 1.5 (UnionTy (IntTy) (FloatTy)))
\ No newline at end of file
diff --git a/tests/expr/t05_union_type_of_single_type.test b/tests/expr/t05_union_type_of_single_type.test
index 7df926e8..b22ddb77 100644
--- a/tests/expr/t05_union_type_of_single_type.test
+++ b/tests/expr/t05_union_type_of_single_type.test
@@ -1,5 +1,4 @@
 discard """
   output: "100 : (UnionTy (IntTy))"
 """
-(ProcDecl (Ident "a") (UnionTy (IntTy)) (Params)
-  (Return (IntVal 100)))
+(As 100 (UnionTy (IntTy)))
diff --git a/tests/expr/t06_union_duplicate_types.test b/tests/expr/t06_union_duplicate_types.test
index a3e1f445..07b4bff8 100644
--- a/tests/expr/t06_union_duplicate_types.test
+++ b/tests/expr/t06_union_duplicate_types.test
@@ -1,5 +1,4 @@
 discard """
   reject: true
 """
-(ProcDecl (Ident "a") (UnionTy (IntTy) (IntTy)) (Params)
-  (Return (IntVal 100)))
\ No newline at end of file
+(As 100 (UnionTy (IntTy) (IntTy)))
diff --git a/tests/expr/t08_if_expr_unify_type_1.test b/tests/expr/t08_if_expr_unify_type_1.test
index b2a9c01a..13bcde44 100644
--- a/tests/expr/t08_if_expr_unify_type_1.test
+++ b/tests/expr/t08_if_expr_unify_type_1.test
@@ -1,13 +1,7 @@
 discard """
   description: "Ensure types unify for an if-expression"
+  output: "0.5 : (UnionTy (IntTy) (FloatTy))"
 """
-(Module
-  (ProcDecl (Ident "p") (UnionTy (IntTy) (FloatTy)) (Params)
-    (Return (IntVal 100)))
-  
-  (ProcDecl (Ident "test") (UnionTy (IntTy) (FloatTy)) (Params)
-    (Return
-      (If (Ident "true")
-        (FloatVal 0.5)
-        (Call (Ident "p")))))
-)
\ No newline at end of file
+(If (Ident "true")
+  (FloatVal 0.5)
+  (As (IntVal 100) (UnionTy (IntTy) (FloatTy)))))
diff --git a/tests/expr/t08_if_expr_unify_type_2.test b/tests/expr/t08_if_expr_unify_type_2.test
index 84b01fc7..89af6497 100644
--- a/tests/expr/t08_if_expr_unify_type_2.test
+++ b/tests/expr/t08_if_expr_unify_type_2.test
@@ -1,13 +1,7 @@
 discard """
   description: "Ensure types unify for an if-expression"
+  output: "0.5 : (UnionTy (IntTy) (FloatTy))"
 """
-(Module
-  (ProcDecl (Ident "p") (UnionTy (IntTy) (FloatTy)) (Params)
-    (Return (IntVal 100)))
-
-  (ProcDecl (Ident "test") (UnionTy (IntTy) (FloatTy)) (Params)
-    (Return
-      (If (Ident "false")
-        (Call (Ident "p"))
-        (FloatVal 0.5))))
-)
\ No newline at end of file
+(If (Ident "false")
+  (As (IntVal 100) (UnionTy (IntTy) (FloatTy)))
+  (FloatVal 0.5))))
\ No newline at end of file
diff --git a/tests/expr/t09_asgn_compatible_type.test b/tests/expr/t09_asgn_compatible_type.test
index a0f74f36..177017a1 100644
--- a/tests/expr/t09_asgn_compatible_type.test
+++ b/tests/expr/t09_asgn_compatible_type.test
@@ -1,12 +1,8 @@
 discard """
   output: "1.5 : (UnionTy (IntTy) (FloatTy))"
 """
-(Module
-  (ProcDecl (Ident "union") (UnionTy (IntTy) (FloatTy)) (Params)
-    (Return (IntVal 100)))
-
-  (ProcDecl (Ident "test") (UnionTy (IntTy) (FloatTy)) (Params)
-    (Exprs
-      (Decl (Ident "x") (Call (Ident "union")))
-      (Asgn (Ident "x") (FloatVal 1.5))
-      (Return (Ident "x")))))
+(Exprs
+  (Decl (Ident "x")
+    (As (IntVal 100) (UnionTy (IntTy) (FloatTy))))
+  (Asgn (Ident "x") (FloatVal 1.5))
+  (Ident "x"))
diff --git a/tests/expr/t10_asgn_lhs_must_be_lvalue_11_error.test b/tests/expr/t10_asgn_lhs_must_be_lvalue_11_error.test
new file mode 100644
index 00000000..023cb640
--- /dev/null
+++ b/tests/expr/t10_asgn_lhs_must_be_lvalue_11_error.test
@@ -0,0 +1,7 @@
+discard """
+  description: "An `As` expression is not an l-value expression"
+  reject: true
+"""
+(Exprs
+  (Decl x 1)
+  (Asgn (As x (IntTy)) 4))
diff --git a/tests/expr/t12_as_copies.test b/tests/expr/t12_as_copies.test
new file mode 100644
index 00000000..08bd4706
--- /dev/null
+++ b/tests/expr/t12_as_copies.test
@@ -0,0 +1,14 @@
+discard """
+  description: "
+    `As` is not an lvalue expression. The sub-expression is fully evaluated
+    early.
+  "
+  output: "1 : (IntTy)"
+"""
+(Exprs
+  (Decl x (Seq (IntTy) 1))
+  (At
+    (As x (SeqTy (IntTy)))
+    (Exprs
+      (Asgn x (Seq (IntTy) 2))
+      0)))
diff --git a/tests/expr/t17_seq_copy_3.test b/tests/expr/t17_seq_copy_3.test
index 339a2274..093c6db2 100644
--- a/tests/expr/t17_seq_copy_3.test
+++ b/tests/expr/t17_seq_copy_3.test
@@ -1,13 +1,10 @@
 discard """
   output: "(array 1 2 3) : (UnionTy (IntTy) (SeqTy (IntTy)))"
 """
-(Module
-  (ProcDecl union (UnionTy (IntTy) (SeqTy (IntTy))) (Params)
-    (Return 1))
-  (ProcDecl main (UnionTy (IntTy) (SeqTy (IntTy))) (Params)
-    (Exprs
-      (Decl x (Seq (IntTy) 1 2 3))
-      (Decl y (Call union))
-      (Asgn y x)
-      (Asgn (At x 0) 4)
-      (Return y))))
+(Exprs
+  (Decl x (Seq (IntTy) 1 2 3))
+  (Decl y
+    (As 1 (UnionTy (IntTy) (SeqTy (IntTy)))))
+  (Asgn y x)
+  (Asgn (At x 0) 4)
+  y)
diff --git a/tests/expr/t17_seq_nested_copy_3.test b/tests/expr/t17_seq_nested_copy_3.test
index 5412ac1a..121a2db9 100644
--- a/tests/expr/t17_seq_nested_copy_3.test
+++ b/tests/expr/t17_seq_nested_copy_3.test
@@ -2,11 +2,9 @@ discard """
   description: "Sequences part of unions are fully copied when the union is"
   output: "(array 1 2 3) : (UnionTy (IntTy) (SeqTy (IntTy)))"
 """
-(Module
-  (ProcDecl union (UnionTy (IntTy) (SeqTy (IntTy))) (Params)
-    (Return (Seq (IntTy) 1 2 3)))
-  (ProcDecl main (UnionTy (IntTy) (SeqTy (IntTy))) (Params)
-    (Exprs
-      (Decl x (Call union))
-      (Decl y x)
-      (Return y))))
+(Exprs
+  (Decl x
+    (As (Seq (IntTy) 1 2 3)
+      (UnionTy (IntTy) (SeqTy (IntTy)))))
+  (Decl y x)
+  y)