Add @take and @drop macros

NEWS.md

@@ -1,3 +1,7 @@
+# Query.jl v0.9.0 Release Notes
+* Add @take and @drop standalone macros
+* Fix some test bugs
+
 # Query.jl v0.8.0 Release Notes
 * Add @groupjoin, @join and @mapmany standalone macros
 * Move backend code to QueryOperators.jl

docs/src/experimental.md

@@ -7,7 +7,7 @@ Query.jl. At the same time any feedback on these features would be
 especially welcome.
 
 The `@map`, `@filter`, `@groupby`, `@orderby` (and various variants),
-`@groupjoin`, `@join` and `@mapmany` commands can be used in standalone
+`@groupjoin`, `@join`, `@mapmany`, `@take` and `@drop` commands can be used in standalone
 versions. Those standalone versions are especially convenient in
 combination with the pipe syntax in julia. Here is an example that
 demonstrates their use:
@@ -124,6 +124,14 @@ be an anonymous function that takes one argument and returns a collection.
 `result_selector` must be an anonymous function that takes two arguments.
 It will be applied to each element of the intermediate collection.
 
+### The `@take` command
+
+The `@take` command has the form `@take(source, n)`. `source` can be any source that can be queried. `n` must be an integer, and it specifies how many elements from the beginning of the source should be kept.
+
+### The `@drop` command
+
+The `@drop` command has the form `@drop(source, n)`. `source` can be any source that can be queried. `n` must be an integer, and it specifies how many elements from the beginning of the source should be dropped from the results.
+
 ## The `..` syntax
 
 The syntax `a..b` is translated into `map(i->i.b, a)` in any query

src/Query.jl

@@ -24,7 +24,7 @@ import QueryOperators: QueryProvider
 export @from, @query, @count, Grouping, @NT
 
 export @map, @filter, @groupby, @orderby, @orderby_descending,
-	@thenby, @thenby_descending, @groupjoin, @join, @mapmany
+	@thenby, @thenby_descending, @groupjoin, @join, @mapmany, @take, @drop
 
 include("query_utils.jl")
 include("query_translation.jl")

src/standalone_query_macros.jl

@@ -238,3 +238,19 @@ macro filter(f)
         helper_namedtuples_replacement |>
         helper_replace_field_extraction_syntax
 end
+
+macro take(source, n)
+    return :(QueryOperators.take(QueryOperators.query($(esc(source))), $(esc(n))))
+end
+
+macro take(n)
+    return :( i -> QueryOperators.take(QueryOperators.query(i), $(esc(n))))
+end
+
+macro drop(source, n)
+    return :(QueryOperators.drop(QueryOperators.query($(esc(source))), $(esc(n))))
+end
+
+macro drop(n)
+    return :( i -> QueryOperators.drop(QueryOperators.query(i), $(esc(n))))
+end

test/test_standalone.jl

@@ -0,0 +1,47 @@
+using Query
+using DataFrames
+using Base.Test
+
+@testset "Standalone Syntax" begin
+
+@testset "@take operator" begin
+    df = DataFrame(a=[1,2,3], b=[3.,2.,1.], c=["a", "b", "c"])
+
+    df2 = df |> @take(2) |> DataFrame
+
+    @test df2 isa DataFrame
+    @test size(df2) == (2,3)
+    @test df2[:a] == [1,2]
+    @test df2[:b] == [3.,2.]
+    @test df2[:c] == ["a", "b"]
+
+    df2 = DataFrame(@take(df, 2))
+
+    @test df2 isa DataFrame
+    @test size(df2) == (2,3)
+    @test df2[:a] == [1,2]
+    @test df2[:b] == [3.,2.]
+    @test df2[:c] == ["a", "b"]
+end
+
+@testset "@drop operator" begin
+    df = DataFrame(a=[1,2,3], b=[3.,2.,1.], c=["a", "b", "c"])
+
+    df2 = df |> @drop(1) |> DataFrame
+
+    @test df2 isa DataFrame
+    @test size(df2) == (2,3)
+    @test df2[:a] == [2,3]
+    @test df2[:b] == [2.,1.]
+    @test df2[:c] == ["b","c"]
+
+    df2 = DataFrame(@drop(df, 1))
+
+    @test df2 isa DataFrame
+    @test size(df2) == (2,3)
+    @test df2[:a] == [2,3]
+    @test df2[:b] == [2.,1.]
+    @test df2[:c] == ["b","c"]
+end
+
+end