Chain.jl

A Julia package for piping a value through a series of transformation expressions using a more convenient syntax than Julia's native piping functionality.

@chain df begin
  dropmissing
  filter(:id => >(6), _)
  groupby(:group)
  combine(:age => sum)
end

df |>
  dropmissing |>
  x -> filter(:id => >(6), x) |>
  x -> groupby(x, :group) |>
  x -> combine(x, :age => sum)

@pipe df |>
  dropmissing |>
  filter(:id => >(6), _)|>
  groupby(_, :group) |>
  combine(_, :age => sum)

@> df begin
  dropmissing
  x -> filter(:id => >(6), x)
  groupby(:group)
  combine(:age => sum)
end

Chain.jl exports the @chain macro.

This macro rewrites a series of expressions into a chain, where the result of one expression is inserted into the next expression following certain rules.

Rule 1

Any expr that is a begin ... end block is flattened. For example, these two pseudocodes are equivalent:

@chain a b c d e f

@chain a begin
    b
    c
    d
end e f

Rule 2

Any expression but the first (in the flattened representation) will have the preceding result inserted as its first argument, unless at least one underscore _ is present. In that case, all underscores will be replaced with the preceding result.

If the expression is a symbol, the symbol is treated equivalently to a function call.

For example, the following code block

@chain begin
    x
    f()
    @g()
    h
    @i
    j(123, _)
    k(_, 123, _)
end

is equivalent to

begin
    local temp1 = f(x)
    local temp2 = @g(temp1)
    local temp3 = h(temp2)
    local temp4 = @i(temp3)
    local temp5 = j(123, temp4)
    local temp6 = k(temp5, 123, temp5)
end

Rule 3

An expression that begins with @aside does not pass its result on to the following expression. Instead, the result of the previous expression will be passed on. This is meant for inspecting the state of the chain. The expression within @aside will not get the previous result auto-inserted, you can use underscores to reference it.

@chain begin
    [1, 2, 3]
    filter(isodd, _)
    @aside @info "There are \$(length(_)) elements after filtering"
    sum
end

Rule 4

It is allowed to start an expression with a variable assignment. In this case, the usual insertion rules apply to the right-hand side of that assignment. This can be used to store intermediate results.

@chain begin
    [1, 2, 3]
    filtered = filter(isodd, _)
    sum
end

filtered == [1, 3]

Rule 5

The @. macro may be used with a symbol to broadcast that function over the preceding result.

@chain begin
    [1, 2, 3]
    @. sqrt
end

is equivalent to

@chain begin
    [1, 2, 3]
    sqrt.(_)
end

• The implicit first argument insertion is useful for many data pipeline scenarios, like groupby, transform and combine in DataFrames.jl
• The _ syntax is there to either increase legibility or to use functions like filter or map which need the previous result as the second argument
• There is no need to type |> over and over
• Any line can be commented out or in without breaking syntax, there is no problem with dangling |> symbols
• The state of the pipeline can easily be checked with the @aside macro
• Flattening of begin ... end blocks allows you to split your chain over multiple lines
• Because everything is just lines with separate expressions and not one huge function call, IDEs can show exactly in which line errors happened
• Pipe is a name defined by Base Julia which can lead to conflicts

An example with a DataFrame:

using DataFrames, Chain

df = DataFrame(group = [1, 2, 1, 2, missing], weight = [1, 3, 5, 7, missing])

result = @chain df begin
    dropmissing
    filter(r -> r.weight < 6, _)
    groupby(:group)
    combine(:weight => sum => :total_weight)
end

The chain block is equivalent to this:

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

The @chain macro replaces all underscores in the following block, unless it encounters another @chain macrocall. In that case, the only underscore that is still replaced by the outer macro is the first argument of the inner @chain. You can use this, for example, in combination with the @aside macro if you need to process a side result further.

@chain df begin
    dropmissing
    filter(r -> r.weight < 6, _)
    @aside @chain _ begin
            select(:group)
            CSV.write("filtered_groups.csv", _)
        end
    groupby(:group)
    combine(:weight => sum => :total_weight)
end