indexing into types

2 files changed, 44 insertions, 4 deletions

diff --git a/alv/type.moon b/alv/type.moon
index ecd1066..408edff 100644
--- a/alv/type.moon
+++ b/alv/type.moon
@@ -4,6 +4,7 @@
 -- @module type
 import opairs from require 'alv.util'
 import result from require 'alv.cycle'
+import Error from require 'alv.error'
 
 shared_shape = (a, b) ->
   for key in pairs a
@@ -35,6 +36,11 @@ class Type
   -- @tparam any b
   -- @treturn bool
 
+  --- index into this type or throw a error.
+  -- @function get
+  -- @tparam any key
+  -- @treturn Type
+
   --- create a `SigStream` of this type.
   -- @tparam ?any init initial value
   -- @treturn SigStream
@@ -67,6 +73,8 @@ class Primitive extends Type
 
   eq: (a, b) => a == b
 
+  get: (key) => error "cannot index into Primitive type '#{@}'"
+
   __eq: (other) => @name == other.name
   __tostring: => @name
 
@@ -93,10 +101,8 @@ class Struct extends Type
         return false
     true
 
-  __eq: (other) => same @types, other.types
-  __tostring: =>
-    inner = table.concat ["#{k}: #{v}" for k, v in opairs @types], ' '
-    "{#{inner}}"
+  get: (key) =>
+    assert @types[key], Error 'index', "#{@} has no '#{key}' key"
 
   --- create a new struct type with a subset of keys.
   project: (keys) =>
@@ -105,6 +111,11 @@ class Struct extends Type
       types[key] = @types[key]
     @@ types
 
+  __eq: (other) => same @types, other.types
+  __tostring: =>
+    inner = table.concat ["#{k}: #{v}" for k, v in opairs @types], ' '
+    "{#{inner}}"
+
   --- instantiate a Primitive type.
   -- @classmethod
   -- @tparam {string=Type} types
@@ -127,6 +138,11 @@ class Array extends Type
         return false
     true
 
+  get: (key) =>
+    assert (type key) == 'number'
+    assert key >= 0 and key < @size, Error 'index', "index '#{key}' out of range!"
+    @type
+
   __eq: (other) => @size == other.size and @type == other.type
   __tostring: => "#{@type}[#{@size}]"
 
diff --git a/spec/type_spec.moon b/spec/type_spec.moon
index b6315f1..2ffdb49 100644
--- a/spec/type_spec.moon
+++ b/spec/type_spec.moon
@@ -36,6 +36,11 @@ describe 'Primitive', ->
     assert.is.true num\eq a, a
     assert.is.false num\eq a, b
 
+  it ':get errors', ->
+    assert.has.error -> bool\get 'hello'
+    assert.has.error -> bool\get 2
+    assert.has.error -> bool\get!
+
 describe 'Array', ->
   vec3 = Array 3, num
   str32 = Array 2, Array 3, str
@@ -69,6 +74,18 @@ describe 'Array', ->
     assert.is.false str32\eq { {'a', 'b', 'c'}, {'d', 'e', 'f'} },
                              { {'a', 'b', 'c'}, {'d', 'e', 'g'} }
 
+  it ':get verifies size range', ->
+    assert.has.error -> vec3\get -1
+    assert.is.equal num, vec3\get 0
+    assert.is.equal num, vec3\get 1
+    assert.is.equal num, vec3\get 2
+    assert.has.error -> vec3\get 3
+
+    assert.is.equal (Array 3, str), str32\get 1
+
+    assert.has.error -> vec3\get!
+    assert.has.error -> vec3\get 'fail'
+
 describe 'Struct', ->
   play = Struct { note: str, dur: num }
   abc = Struct { c: num, b: num, a: num }
@@ -98,6 +115,13 @@ describe 'Struct', ->
     assert.is.true  play\eq { dur: 0.5, note: c }, { dur: 0.5, note: c }
     assert.is.false play\eq { dur: 0.5, note: c }, { dur: 0.5, note: {} }
 
+  it ':get verifies key exists', ->
+    assert.is.equal str, play\get 'note'
+    assert.is.equal num, play\get 'dur'
+    assert.has.error -> play\get!
+    assert.has.error -> play\get ''
+    assert.has.error -> play\get 'something'
+
 describe 'T', ->
   it 'provides shorthand for Primitives', ->
     assert.is.equal num, T.num