Use begin block and allow variable export (#50)

CHANGELOG.md

@@ -0,0 +1,23 @@
+# v0.5
+
+**Breaking**: The `@chain` macro now creates a `begin` block, not a `let` block.
+This means that variables that are assigned within the macro are available outside.
+Technically, situations are imaginable where this could lead to overwritten variables if someone used large expressions with intermediate variable names in begin blocks spliced into the chain.
+It is however quite unlikely for the normal way that `@chain` is intended to be used.
+
+Additionally, it is now possible to use the syntax `variable = some_expression` to make use of the feature that variables can be exported.
+The `some_expression` part is handled exactly like before.
+This enables you to carry parts of a computation forward to a later step in the chain or outside of it:
+
+```julia
+@chain df begin
+    transform(...)
+    select(...)
+    intermediate = subset(...)
+    groupby(...)
+    combine(...)
+    join(intermediate)
+end
+
+@show intermediate
+```

Project.toml

@@ -1,7 +1,7 @@
 name = "Chain"
 uuid = "8be319e6-bccf-4806-a6f7-6fae938471bc"
 authors = ["Julius Krumbiegel"]
-version = "0.4.10"
+version = "0.5.0"
 
 [compat]
 julia = "1"

README.md

@@ -107,14 +107,14 @@ result = @chain df begin
 end
 ```
 
-The pipeless block is equivalent to this:
+The chain block is equivalent to this:
 
 ```julia
-result = let
-    var1 = dropmissing(df)
-    var2 = filter(r -> r.weight < 6, var1)
-    var3 = groupby(var2, :group)
-    var4 = combine(var3, :weight => sum => :total_weight)
+result = begin
+    local var"##1" = dropmissing(df)
+    local var"##2" = filter(r -> r.weight < 6, var"##1")
+    local var"##3" = groupby(var"##2", :group)
+    local var"##4" = combine(var"##3", :weight => sum => :total_weight)
 end
 ```
 
@@ -151,6 +151,21 @@ This works well for short sequences that are still easy to parse visually withou
 @chain 1:10 filter(isodd, _) sum sqrt
 ```
 
+## Variable assignments in the chain
+
+You can prefix any of the expressions that Chain.jl can handle with a variable assignment.
+The previous value will be spliced into the right-hand-side expression and the result will be available afterwards under the chosen variable name.
+
+```julia
+@chain 1:10 begin
+    _ * 3
+    filtered = filter(iseven, _)
+    sum
+end
+
+filtered == [6, 12, 18, 24, 30]
+```
+
 ## The `@aside` macro
 
 For debugging, it's often useful to look at values in the middle of a pipeline.
@@ -172,12 +187,12 @@ end
 Which is again equivalent to this:
 
 ```julia
-result = let
-    var1 = dropmissing(df)
-    var2 = filter(r -> r.weight < 6, var1)
-    var3 = groupby(var2, :group)
-    println("There are $(length(var3)) groups after step 3.")
-    var4 = combine(var3, :weight => sum => :total_weight)
+result = begin
+    local var"##1" = dropmissing(df)
+    local var"##2" = filter(r -> r.weight < 6, var"##1")
+    local var"##3" = groupby(var"##2", :group)
+    println("There are $(length(var"##3")) groups after step 3.")
+    local var"##4" = combine(var"##3", :weight => sum => :total_weight)
 end
 ```
 

src/Chain.jl

@@ -6,8 +6,8 @@ is_aside(x) = false
 is_aside(x::Expr) = x.head == :macrocall && x.args[1] == Symbol("@aside")
 
 
-insert_first_arg(symbol::Symbol, firstarg) = Expr(:call, symbol, firstarg)
-insert_first_arg(any, firstarg) = insertionerror(any)
+insert_first_arg(symbol::Symbol, firstarg; assignment = false) = Expr(:call, symbol, firstarg)
+insert_first_arg(any, firstarg; assignment = false) = insertionerror(any)
 
 function insertionerror(expr)
     error(
@@ -35,12 +35,21 @@ function is_moduled_symbol(e::Expr)
         e.args[2].value isa Symbol
 end
 
-function insert_first_arg(e::Expr, firstarg)
+function insert_first_arg(e::Expr, firstarg; assignment = false)
     head = e.head
     args = e.args
 
+    # variable = ...
+    # set assignment = true and rerun with right hand side
+    if !assignment && head == :(=) && length(args) == 2
+        if !(args[1] isa Symbol)
+            error("You can only use assignment syntax with a Symbol as a variable name, not $(args[1]).")
+        end
+        variable = args[1]
+        righthandside = insert_first_arg(args[2], firstarg; assignment = true)
+        :($variable = $righthandside)
     # Module.SubModule.symbol
-    if is_moduled_symbol(e)
+    elseif is_moduled_symbol(e)
         Expr(:call, e, firstarg)
 
     # f(args...) --> f(firstarg, args...)
@@ -104,7 +113,7 @@ function rewrite(expr, replacement)
             new_expr = insert_first_arg(new_expr, replacement)
         end
         replacement = gensym()
-        new_expr = Expr(Symbol("="), replacement, new_expr)
+        new_expr = :(local $replacement = $new_expr)
     end
 
     (new_expr, replacement)
@@ -188,7 +197,7 @@ function rewrite_chain_block(block)
         # we just do the firstvar transformation for the first non LineNumberNode
         # we encounter
         if !(did_first || expr isa LineNumberNode)
-            expr = Expr(Symbol("="), firstvar, expr)
+            expr = :(local $firstvar = $expr)
             did_first = true
             push!(rewritten_exprs, expr)
             continue
@@ -198,7 +207,7 @@ function rewrite_chain_block(block)
         push!(rewritten_exprs, rewritten)
     end
 
-    result = Expr(:let, Expr(:block), Expr(:block, rewritten_exprs..., replacement))
+    result = Expr(:block, rewritten_exprs..., replacement)
 
     :($(esc(result)))
 end

test/runtests.jl

@@ -109,15 +109,6 @@ end
             sum
         end
     end)
-
-    # variable defined in chain block doesn't leak out
-    z = @chain [1, 2, 3] begin
-        @aside inside_var = 5
-        @aside @test inside_var == 5
-        sum(_) + inside_var
-    end
-    @test z == 11
-    @test_throws UndefVarError inside_var
 end
 
 @testset "nested chains" begin
@@ -441,4 +432,56 @@ end
     @test 36 == @chain 1:3 begin
         @chain _ sum _ ^ 2
     end
-end
+end
+
+@testset "variable assignment syntax" begin
+    result = @chain 1:10 begin
+        x = filter(iseven, _)
+        y = sum
+        sqrt
+    end
+    @test x == filter(iseven, 1:10)
+    @test y == sum(x)
+    @test result == sqrt(y)
+end
+
+module TestModule
+    using Chain
+end
+
+@testset "no variable leaks" begin
+
+    allnames() = Set(names(TestModule, all = true))
+    _names = allnames()
+
+    TestModule.eval(quote
+        @chain 1:10 begin
+            sum
+            sqrt
+        end
+    end)
+
+    @test setdiff(allnames(), _names) == Set()
+
+    TestModule.eval(quote
+        @chain begin
+            1:10
+            sum(_)
+            sqrt(_)
+        end
+    end)
+
+    @test setdiff(allnames(), _names) == Set()
+
+    TestModule.eval(quote
+        @chain begin
+            1:10
+            x = sum(_)
+            y = sqrt(_)
+        end
+    end)
+
+    @test setdiff(allnames(), _names) == Set([:x, :y])
+end
+
+