table comprehensions support single expression that returns a pair

docs/reference.md

@@ -436,6 +436,18 @@ for a few numbers.
     sqrts = {i, math.sqrt i for i in *numbers}
     ```
 
+The key-value tuple in a table comprehension can also come from a single
+expression, in which case the expression should return two values. The
+first is used as the key and the second is used as the value:
+
+In this example we convert an array of pairs to a table where the first item in
+the pair is the key and the second is the value.
+
+    ```moon
+    tuples = {{"hello", "world"}, {"foo", "bar"}}
+    tbl = {unpack tuple for tuple in *tuples}
+    ```
+
 ### Slicing
 
 A special syntax is provided to restrict the items that are iterated over when
@@ -480,7 +492,7 @@ There are two for loop forms, just like in Lua. A numeric one and a generic one:
       print key, value
     ```
     
-The slicing and `*` operators can be used, just like with table comprehensions:
+The slicing and `*` operators can be used, just like with comprehensions:
 
     ```moon
     for item in *items[2,4]

moonscript/parse.lua

@@ -413,7 +413,7 @@ local build_grammar = wrap_env(function()
 
 		Comprehension = sym"[" * Exp * CompInner * sym"]" / mark"comprehension",
 
-		TblComprehension = sym"{" * Exp * (sym"," * Exp)^-1 * CompInner * sym"}" / mark"tblcomprehension",
+		TblComprehension = sym"{" * Ct(Exp * (sym"," * Exp)^-1) * CompInner * sym"}" / mark"tblcomprehension",
 
 		CompInner = Ct(CompFor * CompClause^0),
 		CompFor = key"for" * Ct(NameList) * key"in" * (sym"*" * Exp / mark"unpack" + Exp) / mark"for",

moonscript/transform.lua

@@ -1231,8 +1231,11 @@ Value = Transformer({
     return a:wrap(node)
   end,
   tblcomprehension = function(self, node)
-    local _, key_exp, value_exp, clauses = unpack(node)
+    local _, explist, clauses = unpack(node)
+    local key_exp, value_exp = unpack(explist)
     local accum = NameProxy("tbl")
+    local inner
+    if value_exp then
       local dest = build.chain({
         base = accum,
         {
@@ -1240,12 +1243,34 @@ Value = Transformer({
           key_exp
         }
       })
-    local inner = build.assign_one(dest, value_exp)
+      inner = {
+        build.assign_one(dest, value_exp)
+      }
+    else
+      local key_name, val_name = NameProxy("key"), NameProxy("val")
+      local dest = build.chain({
+        base = accum,
+        {
+          "index",
+          key_name
+        }
+      })
+      inner = {
+        build.assign({
+          names = {
+            key_name,
+            val_name
+          },
+          values = {
+            key_exp
+          }
+        }),
+        build.assign_one(dest, val_name)
+      }
+    end
     return build.block_exp({
       build.assign_one(accum, build.table()),
-      construct_comprehension({
+      construct_comprehension(inner, clauses),
-        inner
-      }, clauses),
       accum
     })
   end,

moonscript/transform.moon

@@ -654,15 +654,27 @@ Value = Transformer {
     a\wrap node
 
   tblcomprehension: (node) =>
-    _, key_exp, value_exp, clauses = unpack node
+    _, explist, clauses = unpack node
+    key_exp, value_exp = unpack explist
 
     accum = NameProxy "tbl"
+
+    inner = if value_exp
       dest = build.chain { base: accum, {"index", key_exp} }
-    inner = build.assign_one dest, value_exp
+      { build.assign_one dest, value_exp }
+    else
+      -- If we only have single expression then
+      -- unpack the result into key and value
+      key_name, val_name = NameProxy"key", NameProxy"val"
+      dest = build.chain { base: accum, {"index", key_name} }
+      {
+        build.assign names: {key_name, val_name}, values: {key_exp}
+        build.assign_one dest, val_name
+      }
 
     build.block_exp {
       build.assign_one accum, build.table!
-      construct_comprehension {inner}, clauses
+      construct_comprehension inner, clauses
       accum
     }
 

tests/inputs/comprehension.moon

@@ -1,4 +1,6 @@
 
+-- see lists.moon for list comprehension tests
+
 items = {1,2,3,4,5,6}
 out = {k,k*2 for k in items}
 
@@ -7,3 +9,11 @@ x = hello: "world", okay: 2323
 
 copy  = {k,v for k,v in pairs x when k != "okay"}
 
+--
+
+{ unpack(x) for x in yes }
+{ unpack(x) for x in *yes }
+
+{ xxxx for x in yes }
+{ unpack [a*i for i, a in ipairs x] for x in *{{1,2}, {3,4}} }
+

tests/outputs/comprehension.lua

@@ -26,3 +26,56 @@ local copy = (function()
   end
   return _tbl_0
 end)()
+local _ = (function()
+  local _tbl_0 = { }
+  for x in yes do
+    local _key_0, _val_0 = unpack(x)
+    _tbl_0[_key_0] = _val_0
+  end
+  return _tbl_0
+end)()
+_ = (function()
+  local _tbl_0 = { }
+  local _list_0 = yes
+  for _index_0 = 1, #_list_0 do
+    x = _list_0[_index_0]
+    local _key_0, _val_0 = unpack(x)
+    _tbl_0[_key_0] = _val_0
+  end
+  return _tbl_0
+end)()
+_ = (function()
+  local _tbl_0 = { }
+  for x in yes do
+    local _key_0, _val_0 = xxxx
+    _tbl_0[_key_0] = _val_0
+  end
+  return _tbl_0
+end)()
+_ = (function()
+  local _tbl_0 = { }
+  local _list_0 = {
+    {
+      1,
+      2
+    },
+    {
+      3,
+      4
+    }
+  }
+  for _index_0 = 1, #_list_0 do
+    x = _list_0[_index_0]
+    local _key_0, _val_0 = unpack((function()
+      local _accum_0 = { }
+      local _len_0 = 0
+      for i, a in ipairs(x) do
+        _len_0 = _len_0 + 1
+        _accum_0[_len_0] = a * i
+      end
+      return _accum_0
+    end)())
+    _tbl_0[_key_0] = _val_0
+  end
+  return _tbl_0
+end)()