diff --git a/core/Indexable.savi b/core/Indexable.savi
index 1c8b8c9b..721321c2 100644
--- a/core/Indexable.savi
+++ b/core/Indexable.savi
@@ -1,4 +1,6 @@
+:: An collection of elements of type A that can be accessed via numeric index.
 :trait box Indexable(A)
+  :fun size USize
   :fun "[]!"(index USize) (@->A)'aliased
 
   //--
@@ -9,6 +11,16 @@
     stride USize = 1
   ) None
     :yields ((@->A)'aliased, USize)
+    index = from
+    to = to.at_most(@size)
+    stride = stride.at_least(1)
+    while index < to (
+      try (
+        value = @[index]!
+        yield (--value, index)
+      )
+      index = try (index +! stride | return)
+    )
 
   :fun each(
     from USize = 0
@@ -95,6 +107,15 @@
     stride USize = 1
   ) None
     :yields ((@->A)'aliased, USize)
+    index = from.at_most(try (@size -! 1 | return))
+    stride = stride.at_least(1)
+    while index >= to (
+      try (
+        value = @[index]!
+        yield (--value, index)
+      )
+      index = try (index -! stride | return)
+    )
 
   :fun reverse_each(
     from = USize.max_value
@@ -129,6 +150,18 @@
 
   //--
 
+  :fun first! @->(A'aliased)
+    @[0]!
+
+  :fun last! @->(A'aliased)
+    @reverse_each_with_index -> (value, index |
+      return value
+      None // TODO: this should not be needed
+    )
+    error!
+
+  //--
+
   :fun select(
     from USize = 0
     to = USize.max_value
diff --git a/spec/core/Indexable.Spec.savi b/spec/core/Indexable.Spec.savi
index 295ad815..f1e2418a 100644
--- a/spec/core/Indexable.Spec.savi
+++ b/spec/core/Indexable.Spec.savi
@@ -1,121 +1,122 @@
+// A little custom class to showcase the minimum trait implementation.
+// All other Indexable methods will be based on this minimal implementation.
+:class _ASCIILettersExample
+  :is Indexable(String)
+
+  :fun size USize: 26
+  :fun "[]!"(index USize)
+    error! if index >= @size
+    "\((index + 'a').format.printable_ascii)"
+
+:class _ArrayWrapperExample(T val) // TODO: val constraint should not be required
+  :is Indexable(T)
+  :let array Array(T)
+  :new (@array)
+
+  :fun size: @array.size
+  :fun "[]!"(index USize): @array[index]!
+
 :class Savi.Indexable.Spec
   :is Spec
   :const describes: "Indexable"
 
   :it "yields each element"
     array Array(String) = []
-    ["foo", "bar", "baz"].each -> (string | array << string)
-    assert: array == ["foo", "bar", "baz"]
+    _ASCIILettersExample.new.each -> (string | array << string)
+    assert: array == [
+      "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m"
+      "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
+    ]
 
   :it "yields each element of a subslice"
     array Array(String) = []
-    ["a", "b", "c", "d", "e", "f"].each(1, 5) -> (string | array << string)
+    _ASCIILettersExample.new.each(1, 5) -> (string | array << string)
     assert: array == ["b", "c", "d", "e"]
 
   :it "yields each element along with the index"
     array_a Array(String) = []
     array_b Array(USize) = []
-    ["foo", "bar", "baz"].each_with_index -> (string, index |
+    _ASCIILettersExample.new.each_with_index -> (string, index |
       array_a << string
       array_b << index
     )
-    assert: array_a == ["foo", "bar", "baz"]
-    assert: array_b == [0, 1, 2]
+    assert: array_a == [
+      "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m"
+      "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
+    ]
+    assert: array_b == [
+      0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
+      13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25
+    ]
 
   :it "yields each element, in reverse"
     array Array(String) = []
-    ["foo", "bar", "baz"].reverse_each -> (string | array << string)
-    assert: array == ["baz", "bar", "foo"]
+    _ASCIILettersExample.new.reverse_each -> (string | array << string)
+    assert: array == [
+      "z", "y", "x", "w", "v", "u", "t", "s", "r", "q", "p", "o", "n"
+      "m", "l", "k", "j", "i", "h", "g", "f", "e", "d", "c", "b", "a"
+    ]
 
   :it "yields each element, in reverse, along with the index"
     array_a Array(String) = []
     array_b Array(USize) = []
-    ["foo", "bar", "baz"].reverse_each_with_index -> (string, index |
+    _ASCIILettersExample.new.reverse_each_with_index -> (string, index |
       array_a << string
       array_b << index
     )
-    assert: array_b == [2, 1, 0]
-    assert:
-      array_a == ["baz", "bar", "foo"]
-
-  :it "yields each element, stopping early if the criteria is met"
-    array Array(String) = []
-    early_stop = ["foo", "bar", "baz"].each_until -> (string |
-      array << string
-      string == "bar"
-    )
-    assert: early_stop
-    assert: array == ["foo", "bar"]
-
-    array.clear
-    early_stop = ["foo", "bar", "baz"].each_until -> (string |
-      array << string
-      string == "bard"
-    )
-    assert: early_stop.is_false
-    assert: array == ["foo", "bar", "baz"]
-
-  :it "yields each element of a subslice, stopping early if the criteria is met"
-    array Array(String) = []
-    early_stop = ["a", "b", "c", "d", "e", "f"].each_until(1, 5) -> (string |
-      array << string
-      string == "d"
-    )
-    assert: early_stop
-    assert: array == ["b", "c", "d"]
-
-    array.clear
-    early_stop = ["a", "b", "c", "d", "e", "f"].each_until(1, 5) -> (string |
-      array << string
-      string == "z"
-    )
-    assert: early_stop.is_false
-    assert: array == ["b", "c", "d", "e"]
+    assert: array_a == [
+      "z", "y", "x", "w", "v", "u", "t", "s", "r", "q", "p", "o", "n"
+      "m", "l", "k", "j", "i", "h", "g", "f", "e", "d", "c", "b", "a"
+    ]
+    assert: array_b == [
+      25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13,
+      12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0
+    ]
 
   :it "returns True if any element meets the criteria"
-    array Array(U8) = [11, 22, 33, 44, 36, 27, 18]
+    array = _ArrayWrapperExample(U8).new([11, 22, 33, 44, 36, 27, 18])
     assert: array.has_any -> (num | num > 30)
     assert: array.has_any -> (num | num > 50).is_false
 
   :it "returns True if all elements meet the criteria"
-    array Array(U8) = [11, 22, 33, 44, 36, 27, 18]
+    array = _ArrayWrapperExample(U8).new([11, 22, 33, 44, 36, 27, 18])
     assert: array.has_all -> (num | num > 10)
     assert: array.has_all -> (num | num > 30).is_false
 
   :it "finds the first element that meets the criteria"
-    array Array(U8) = [11, 22, 33, 44, 36, 27, 18]
+    array = _ArrayWrapperExample(U8).new([11, 22, 33, 44, 36, 27, 18])
     assert: array.find! -> (num | num > 30) == 33
     assert error: array.find! -> (num | num > 50)
 
   :it "finds the first index that meets the criteria"
-    array Array(U8) = [11, 22, 33, 44, 36, 27, 18]
+    array = _ArrayWrapperExample(U8).new([11, 22, 33, 44, 36, 27, 18])
     assert: array.find_index! -> (num | num > 30) == 2
     assert error: array.find_index! -> (num | num > 50)
 
   :it "finds, starting from the end, the first element that meets the criteria"
-    array Array(U8) = [11, 22, 33, 44, 36, 27, 18]
+    array = _ArrayWrapperExample(U8).new([11, 22, 33, 44, 36, 27, 18])
     assert: array.reverse_find! -> (num | num > 30) == 36
     assert error: array.reverse_find! -> (num | num > 50)
 
   :it "finds, starting from the end, the first index that meets the criteria"
-    array Array(U8) = [11, 22, 33, 44, 36, 27, 18]
+    array = _ArrayWrapperExample(U8).new([11, 22, 33, 44, 36, 27, 18])
     assert: array.reverse_find_index! -> (num | num > 30) == 4
     assert error: array.reverse_find_index! -> (num | num > 50)
 
   :it "selects those elements that meet the criteria"
-    array Array(U8) = [11, 22, 33, 44, 36, 27, 18]
+    array = _ArrayWrapperExample(U8).new([11, 22, 33, 44, 36, 27, 18])
     selected = array.select -> (num | num < 30)
     assert: selected == [11, 22, 27, 18]
 
   :it "rejects those elements that do not meet the criteria"
-    array Array(U8) = [1, 2, 3, 4, 5]
+    array = _ArrayWrapperExample(U8).new([1, 2, 3, 4, 5])
     odds = array.reject -> (num | num % 2 == 0)
     assert: odds == [1, 3, 5]
 
   :it "rejects nothing from an empty array"
-    array Array(U8) = []
-    assert: array.reject -> (num | num % 2 == 0) == array
+    array = _ArrayWrapperExample(U8).new([])
+    assert: array.reject -> (num | num % 2 == 0) == []
 
   :it "rejects nothing if criteria is always false"
-    array Array(U8) = [1, 2, 3]
-    assert: array.reject -> (num | False) == array
+    array = _ArrayWrapperExample(U8).new([1, 2, 3])
+    assert: array.reject -> (num | False) == [1, 2, 3]
diff --git a/src/savi/compiler/populate.cr b/src/savi/compiler/populate.cr
index ffbb1fb2..57fb6ac0 100644
--- a/src/savi/compiler/populate.cr
+++ b/src/savi/compiler/populate.cr
@@ -205,11 +205,17 @@ class Savi::Compiler::Populate
         visitor = self
         params = f.params.try(&.accept(ctx, visitor))
         ret = f.ret.try(&.accept(ctx, visitor))
+        error_out = f.error_out.try(&.accept(ctx, visitor))
+        yield_out = f.yield_out.try(&.accept(ctx, visitor))
+        yield_in = f.yield_in.try(&.accept(ctx, visitor))
         body = f.body.try(&.accept(ctx, visitor))
 
         f = f.dup
         f.params = params
         f.ret = ret
+        f.error_out = error_out
+        f.yield_out = yield_out
+        f.yield_in = yield_in
         f.body = body
         f
       }