Another attempt at an astable flag

Project.toml

@@ -6,14 +6,15 @@ version = "0.9.1"
 Chain = "8be319e6-bccf-4806-a6f7-6fae938471bc"
 DataFrames = "a93c6f00-e57d-5684-b7b6-d8193f3e46c0"
 MacroTools = "1914dd2f-81c6-5fcd-8719-6d5c9610ff09"
+OrderedCollections = "bac558e1-5e72-5ebc-8fee-abe8a469f55d"
 Reexport = "189a3867-3050-52da-a836-e630ba90ab69"
 
 [compat]
+Chain = "0.4"
 DataFrames = "1"
 MacroTools = "0.5"
 Reexport = "0.2, 1"
 julia = "1"
-Chain = "0.4"
 
 [extras]
 CategoricalArrays = "324d7699-5711-5eae-9e2f-1d82baa6b597"

docs/src/index.md

@@ -22,6 +22,7 @@ In addition, DataFramesMeta provides
   convenient syntax. 
 * `@byrow` for applying functions to each row of a data frame (only supported inside other macros).
 * `@passmissing` for propagating missing values inside row-wise DataFramesMeta.jl transformations.
+* `@astable` to create multiple columns within a single transformation.
 * `@chain`, from [Chain.jl](https://github.com/jkrumbiegel/Chain.jl) for piping the above macros together, similar to [magrittr](https://cran.r-project.org/web/packages/magrittr/vignettes/magrittr.html)'s
   `%>%` in R. 
 
@@ -396,11 +397,38 @@ julia> @rtransform df @passmissing x = parse(Int, :x_str)
    3 │ missing  missing
 ```
 
+## Creating multiple columns at once with `@astable`
+
+Often new variables may depend on the same intermediate calculations. `@astable` makes it easy to create multiple
+new variables in the same operation, yet have them share
+information. 
+
+In a single block, all assignments of the form `:y = f(:x)` 
+or `$y = f(:x)` at the top-level generate new columns. In the 2nd example, `y`
+must be a string or `Symbol`. 
+
+```
+julia> df = DataFrame(a = [1, 2, 3], b = [400, 500, 600]);
+
+julia> @transform df @astable begin 
+           ex = extrema(:b)
+           :b_first = :b .- first(ex)
+           :b_last = :b .- last(ex)
+       end
+3×4 DataFrame
+ Row │ a      b      b_first  b_last 
+     │ Int64  Int64  Int64    Int64  
+─────┼───────────────────────────────
+   1 │     1    400        0    -200
+   2 │     2    500      100    -100
+   3 │     3    600      200       0
+```
+
+
 ## [Working with column names programmatically with `$`](@id dollar)
 
 DataFramesMeta provides the special syntax `$` for referring to 
-columns in a data frame via a `Symbol`, string, or column position as either
-a literal or a variable. 
+columns in a data frame via a `Symbol`, string, or column position as either a literal or a variable. 
 
 ```julia
 df = DataFrame(A = 1:3, B = [2, 1, 2])

src/DataFramesMeta.jl

@@ -4,6 +4,8 @@ using Reexport
 
 using MacroTools
 
+using OrderedCollections: OrderedCollections
+
 @reexport using DataFrames
 
 @reexport using Chain
@@ -16,12 +18,13 @@ export @with,
        @transform, @select, @transform!, @select!,
        @rtransform, @rselect, @rtransform!, @rselect!,
        @eachrow, @eachrow!,
-       @byrow, @passmissing,
+       @byrow, @passmissing, @astable,
        @based_on, @where # deprecated
 
 const DOLLAR = raw"$"
 
 include("parsing.jl")
+include("parsing_astable.jl")
 include("macros.jl")
 include("linqmacro.jl")
 include("eachrow.jl")

src/macros.jl

@@ -282,11 +282,10 @@ macro byrow(args...)
     throw(ArgumentError("@byrow is deprecated outside of DataFramesMeta macros."))
 end
 
-
 """
-    passmissing(args...)
+    @passmissing(args...)
 
-Propograte missing values inside DataFramesMeta.jl macros.
+Propagrate missing values inside DataFramesMeta.jl macros.
 
 
 `@passmissing` is not a "real" Julia macro but rather serves as a "flag"
@@ -350,6 +349,156 @@ macro passmissing(args...)
     throw(ArgumentError("@passmissing only works inside DataFramesMeta macros."))
 end
 
+global astable_docstring_snippet = """
+    Transformations can also use the macro-flag `@astable` for creating multiple
+    new columns at once and letting transformations share the same name-space.
+    See `? @astable` for more details.
+    """
+
+"""
+    @astable(args...)
+
+Return a `NamedTuple` from a single transformation inside the DataFramesMeta.jl
+macros, `@select`, `@transform`, and their mutating and row-wise equivalents.
+
+`@astable` acts on a single block. It works through all top-level expressions
+and collects all such expressions of the form `:y = ...` or `$(DOLLAR)y = ...`, i.e. assignments to a
+`Symbol` or an escaped column identifier, which is a syntax error outside of
+DataFramesMeta.jl macros. At the end of the expression, all assignments are collected
+into a `NamedTuple` to be used with the `AsTable` destination in the DataFrames.jl
+transformation mini-language.
+
+Concretely, the expressions
+
+```
+df = DataFrame(a = 1)
+
+@rtransform df @astable begin
+    :x = 1
+    y = 50
+    :z = :x + y + :a
+end
+```
+
+become the pair
+
+```
+function f(a)
+    x_t = 1
+    y = 50
+    z_t = x_t + y + a
+
+    (; x = x_t, z = z_t)
+end
+
+transform(df, [:a] => ByRow(f) => AsTable)
+```
+
+`@astable` has two major advantages at the cost of increasing complexity.
+First, `@astable` makes it easy to create multiple columns from a single
+transformation, which share a scope. For example, `@astable` allows
+for the following (where `:x` and `:x_2` exist in the data frame already).
+
+```
+@transform df @astable begin
+    m = mean(:x)
+    :x_demeaned = :x .- m
+    :x2_demeaned = :x2 .- m
+end
+```
+
+The creation of `:x_demeaned` and `:x2_demeaned` both share the variable `m`,
+which does not need to be calculated twice.
+
+Second, `@astable` is useful when performing intermediate calculations
+and storing their results in new columns. For example, the following fails.
+
+```
+@rtransform df begin
+    :new_col_1 = :x + :y
+    :new_col_2 = :new_col_1 + :z
+end
+```
+
+This because DataFrames.jl does not guarantee sequential evaluation of
+transformations. `@astable` solves this problem
+
+@rtransform df @astable begin
+    :new_col_1 = :x + :y
+    :new_col_2 = :new_col_1 + :z
+end
+
+Column assignment in `@astable` follows similar rules as
+column assignment in other DataFramesMeta.jl macros. The left-
+-hand-side of a column assignment can be either a `Symbol` or any
+expression which evaluates to a `Symbol` or `AbstractString`. For example
+`:y = ...`, and `$(DOLLAR)y = ...` are both valid ways of assigning a new column.
+However unlike other DataFramesMeta.jl macros, multi-column assignments via
+`AsTable` are disallowed. The following will fail.
+
+```
+@transform df @astable begin
+    $AsTable = :x
+end
+```
+
+References to existing columns also follow the same
+rules as other DataFramesMeta.jl macros.
+
+### Examples
+
+```
+julia> df = DataFrame(a = [1, 2, 3], b = [4, 5, 6]);
+
+julia> d = @rtransform df @astable begin
+           :x = 1
+           y = 5
+           :z = :x + y
+       end
+3×4 DataFrame
+ Row │ a      b      x      z
+     │ Int64  Int64  Int64  Int64
+─────┼────────────────────────────
+   1 │     1      4      1      6
+   2 │     2      5      1      6
+   3 │     3      6      1      6
+
+julia> df = DataFrame(a = [1, 1, 2, 2], b = [5, 6, 70, 80]);
+
+julia> @by df :a @astable begin
+            ex = extrema(:b)
+            :min_b = first(ex)
+            :max_b = last(ex)
+       end
+2×3 DataFrame
+ Row │ a      min_b  max_b
+     │ Int64  Int64  Int64
+─────┼─────────────────────
+   1 │     1      5      6
+   2 │     2     70     80
+
+julia> new_col = "New Column";
+
+julia> @rtransform df @astable begin
+           f_a = first(:a)
+           $(DOLLAR)new_col = :a + :b + f_a
+           :y = :a * :b
+       end
+4×4 DataFrame
+ Row │ a      b      New Column  y
+     │ Int64  Int64  Int64       Int64
+─────┼─────────────────────────────────
+   1 │     1      5           7      5
+   2 │     1      6           8      6
+   3 │     2     70          74    140
+   4 │     2     80          84    160
+```
+
+"""
+macro astable(args...)
+    throw(ArgumentError("@astable only works inside DataFramesMeta macros."))
+end
+
 ##############################################################################
 ##
 ## @with
@@ -1097,6 +1246,8 @@ transformations by row, `@transform` allows `@byrow` at the
 beginning of a block of transformations (i.e. `@byrow begin... end`).
 All transformations in the block will operate by row.
 
+$astable_docstring_snippet
+
 ### Examples
 
 ```jldoctest
@@ -1233,6 +1384,8 @@ transform!ations by row, `@transform!` allows `@byrow` at the
 beginning of a block of transform!ations (i.e. `@byrow begin... end`).
 All transform!ations in the block will operate by row.
 
+$astable_docstring_snippet
+
 ### Examples
 
 ```jldoctest
@@ -1345,6 +1498,8 @@ transformations by row, `@select` allows `@byrow` at the
 beginning of a block of selectations (i.e. `@byrow begin... end`).
 All transformations in the block will operate by row.
 
+$astable_docstring_snippet
+
 ### Examples
 
 ```jldoctest
@@ -1465,6 +1620,8 @@ transformations by row, `@select!` allows `@byrow` at the
 beginning of a block of select!ations (i.e. `@byrow begin... end`).
 All transformations in the block will operate by row.
 
+$astable_docstring_snippet
+
 ### Examples
 
 ```jldoctest
@@ -1546,17 +1703,6 @@ function combine_helper(x, args...; deprecation_warning = false)
 
     exprs, outer_flags = create_args_vector(args...)
 
-    fe = first(exprs)
-    if length(exprs) == 1 &&
-        get_column_expr(fe) === nothing &&
-        !(fe.head == :(=) || fe.head == :kw)
-
-        @warn "Returning a Table object from @by and @combine now requires `$(DOLLAR)AsTable` on the LHS."
-
-        lhs = Expr(:$, :AsTable)
-        exprs = ((:($lhs = $fe)),)
-    end
-
     t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
 
     quote
@@ -1592,6 +1738,8 @@ and
 @combine(df, :mx = mean(:x), :sx = std(:x))
 ```
 
+$astable_docstring_snippet
+
 ### Examples
 
 ```julia
@@ -1666,16 +1814,6 @@ end
 function by_helper(x, what, args...)
     # Only allow one argument when returning a Table object
     exprs, outer_flags = create_args_vector(args...)
-    fe = first(exprs)
-    if length(exprs) == 1 &&
-        get_column_expr(fe) === nothing &&
-        !(fe.head == :(=) || fe.head == :kw)
-
-        @warn "Returning a Table object from @by and @combine now requires `\$AsTable` on the LHS."
-
-        lhs = Expr(:$, :AsTable)
-        exprs = ((:($lhs = $fe)),)
-    end
 
     t = (fun_to_vec(ex; gensym_names = false, outer_flags = outer_flags) for ex in exprs)
 
@@ -1718,6 +1856,8 @@ and
 @by(df, :g, mx = mean(:x), sx = std(:x))
 ```
 
+$astable_docstring_snippet
+
 ### Examples
 
 ```julia