Add tests for mutate

src/docstrings.jl

@@ -532,6 +532,33 @@ julia> @chain df begin
    4 │ d         4     14          1         11
    5 │ e         5     15          1         11
 ```
+Note that unlike dplyr, @mutate transformations cannot be applied progressively-meaning you cannot reuse variables within the same @mutate call. This requires the use of separate @mutate calls.
+```
+julia> @chain df begin
+        #=
+        it's tempting to do this:
+
+          @mutate begin
+            b2 = b * 2
+            b3 = b2 * 2
+          end
+
+        but this syntactic sugar isn't supported.
+        use separate @mutate calls instead.
+        =#
+          @mutate b2 = b * 2
+          @mutate b3 = b2 * 2
+        end
+5×5 DataFrame
+ Row │ a     b      c      b2     b3
+     │ Char  Int64  Int64  Int64  Int64
+─────┼──────────────────────────────────
+   1 │ a         1     11      2      4
+   2 │ b         2     12      4      8
+   3 │ c         3     13      6     12
+   4 │ d         4     14      8     16
+   5 │ e         5     15     10     20
+```
 """
 
 const docstring_summarize =

test/runtests.jl

@@ -22,4 +22,5 @@ test_df = DataFrame(
 
 @testset "TidierData" verbose = true begin
     include("test_pivots.jl")
+    include("test_mutate.jl")
 end

test/test_mutate.jl

@@ -0,0 +1,41 @@
+@testset "@mutate()" verbose = true begin
+
+    @testset "empty mutate returns input" begin
+        df = DataFrame(x = 1)
+        gf = @group_by(df, x)
+
+        @test isequal(@mutate(df), df)
+        @test isequal(@mutate(gf), gf)
+
+
+        @test isequal(@mutate(df, []), df)
+        @test isequal(@mutate(gf, []), gf)
+    end
+
+    @testset "length-1 vectors are recycled" begin
+        df = DataFrame(x = 1:4)
+        @test isequal(@mutate(df, y = 1)[!, :y], fill(1, 4))
+        @test_throws "ArgumentError: New columns must have the same length as old columns" @mutate(df, y = 1:2)
+    end
+
+    @testset "mutate supports constants" begin
+        df = DataFrame(x = 1:10, g =  repeat(1:2, inner = 5))
+        y = 1:10
+        z = 1:5
+
+        @test isequal(@mutate(df, y = !!y)[!, :y], y)
+    end
+
+    @testset "mutate works on empty dataframes" begin
+        df = DataFrame()
+        res = @mutate(df)
+        @test isequal(nrow(res), 0)
+        @test isequal(ncol(res), 0)
+
+        res = @mutate(df, x = Int64[])
+        @test isequal(names(res), ["x"])
+        @test isequal(nrow(res), 0)
+        @test isequal(ncol(res), 1)
+    end
+
+end