diff --git a/.JuliaFormatter.toml b/.JuliaFormatter.toml
index 99729e31..faad8fd6 100644
--- a/.JuliaFormatter.toml
+++ b/.JuliaFormatter.toml
@@ -3,6 +3,7 @@ align_matrix = true
 align_pair_arrow = true
 align_struct_field = true
 always_for_in = true
+always_use_return = true
 conditional_to_if = true
 for_in_replacement = "in"
 format_docstrings = false
diff --git a/benchmark/profiling.jl b/benchmark/profiling.jl
index 60b5ab44..af46faf9 100644
--- a/benchmark/profiling.jl
+++ b/benchmark/profiling.jl
@@ -13,10 +13,10 @@ for file in readdir(NORSE_PATH; join = false)
 end
 # Add another line to rep-periods-data.csv and rep-periods-mapping.csv
 open(joinpath(input_dir, "rep-periods-data.csv"), "a") do io
-    println(io, "3,$new_rp_length,0.1")
+    return println(io, "3,$new_rp_length,0.1")
 end
 open(joinpath(input_dir, "rep-periods-mapping.csv"), "a") do io
-    println(io, "216,3,1")
+    return println(io, "216,3,1")
 end
 # Add profiles to flow and asset
 open(joinpath(input_dir, "flows-profiles.csv"), "a") do io
diff --git a/docs/src/91-developer.md b/docs/src/91-developer.md
index 2963be0a..58be6a92 100644
--- a/docs/src/91-developer.md
+++ b/docs/src/91-developer.md
@@ -198,6 +198,7 @@ We will try to follow these during development and reviews.
   - List obvious objects, e.g., `using JuMP: @variable`, since `@variable` is obviously from JuMP in this context, or `using Graph: SimpleDiGraph`, because it's a constructor with an obvious name.
   - For other objects inside `Package`, use `using Package: Package` and explicitly call `Package.A` to use it, e.g., `DataFrames.groupby`.
   - List all `using` in <src/TulipaEnergyModel.jl>.
+- Explicitly state what a function will `return`, including `return nothing`.
 
 ## Contributing Workflow
 
@@ -441,8 +442,7 @@ See the file <benchmark/profiling.jl> for an example of profiling code.
 When publishing a new version of the model to the Julia Registry, follow this procedure:
 
 > **Note:**
-> To be able to register, you need to be a member of the organisation TulipaEnergy and have your visibility set to public:<br>
-> ![Screenshot of public members of TulipaEnergy on GitHub](./images/PublicMember.png)
+> To be able to register, you need to be a member of the organisation TulipaEnergy and have your visibility set to public:<br> > ![Screenshot of public members of TulipaEnergy on GitHub](./images/PublicMember.png)
 
 1. Click on the `Project.toml` file on GitHub.
 
diff --git a/src/constraints/consumer.jl b/src/constraints/consumer.jl
index da186088..94a2bfcc 100644
--- a/src/constraints/consumer.jl
+++ b/src/constraints/consumer.jl
@@ -19,7 +19,7 @@ function add_consumer_constraints!(model, constraints, graph, sets)
 
     # - Balance constraint (using the lowest temporal resolution)
     df = filter(:asset => ∈(Ac), constraints[:highest_in_out].indices; view = true)
-    model[:consumer_balance] = [
+    return model[:consumer_balance] = [
         @constraint(
             model,
             incoming_flow_highest_in_out_resolution[row.index] -
diff --git a/src/constraints/conversion.jl b/src/constraints/conversion.jl
index 99e2d085..d7f367e1 100644
--- a/src/constraints/conversion.jl
+++ b/src/constraints/conversion.jl
@@ -17,7 +17,7 @@ function add_conversion_constraints!(model, constraints, sets)
     df = filter(:asset => ∈(Acv), constraints[:lowest].indices; view = true)
     incoming = constraints[:lowest].expressions[:incoming]
     outgoing = constraints[:lowest].expressions[:outgoing]
-    model[:conversion_balance] = [
+    return model[:conversion_balance] = [
         @constraint(
             model,
             incoming[row.index] == outgoing[row.index],
diff --git a/src/constraints/energy.jl b/src/constraints/energy.jl
index 39dbdee3..42531807 100644
--- a/src/constraints/energy.jl
+++ b/src/constraints/energy.jl
@@ -29,7 +29,7 @@ function add_energy_constraints!(model, constraints, graph)
     ]
 
     # - Minimum outgoing energy within each period block
-    model[:min_energy_inter_rp] = [
+    return model[:min_energy_inter_rp] = [
         @constraint(
             model,
             constraints[:min_energy_inter_rp].expressions[:outgoing][row.index] ≥
diff --git a/src/constraints/group.jl b/src/constraints/group.jl
index 006d5829..481bb53a 100644
--- a/src/constraints/group.jl
+++ b/src/constraints/group.jl
@@ -42,7 +42,7 @@ function add_group_constraints!(model, variables, graph, sets, groups)
 
     groups_with_min_investment_limit =
         (group for group in groups if !ismissing(group.min_investment_limit))
-    model[:investment_group_min_limit] = [
+    return model[:investment_group_min_limit] = [
         @constraint(
             model,
             investment_group[group] ≥ group.min_investment_limit,
diff --git a/src/constraints/hub.jl b/src/constraints/hub.jl
index ee51e061..e8a842fd 100644
--- a/src/constraints/hub.jl
+++ b/src/constraints/hub.jl
@@ -17,7 +17,7 @@ function add_hub_constraints!(model, constraints, sets)
     outgoing_flow_highest_in_out_resolution = constraints[:highest_in_out].expressions[:outgoing]
     # - Balance constraint (using the lowest temporal resolution)
     df = filter(:asset => ∈(Ah), constraints[:highest_in_out].indices; view = true)
-    model[:hub_balance] = [
+    return model[:hub_balance] = [
         @constraint(
             model,
             incoming_flow_highest_in_out_resolution[row.index] ==
diff --git a/src/constraints/transport.jl b/src/constraints/transport.jl
index 3d2221d1..e2199adb 100644
--- a/src/constraints/transport.jl
+++ b/src/constraints/transport.jl
@@ -71,7 +71,7 @@ function add_transport_constraints!(model, variables, graph, sets)
     ]
 
     # - Min transport flow limit
-    model[:min_transport_flow_limit] = [
+    return model[:min_transport_flow_limit] = [
         @constraint(
             model,
             flow[row.index] ≥ -lower_bound_transport_flow[idx],
diff --git a/src/io.jl b/src/io.jl
index 059673fe..909bdc84 100644
--- a/src/io.jl
+++ b/src/io.jl
@@ -97,7 +97,7 @@ function create_internal_structures(connection)
         @assert table_name in ("asset_$year_prefix", "flow_$year_prefix")
 
         result = _query_data_per_year(table_name, col, year_col; where_pairs...)
-        Dict(row[Symbol(year_col)] => getproperty(row, Symbol(col)) for row in result)
+        return Dict(row[Symbol(year_col)] => getproperty(row, Symbol(col)) for row in result)
     end
 
     _query_data_per_both_years(table_name, col; where_pairs...) = begin
@@ -331,7 +331,7 @@ function save_solution_to_file(output_folder, energy_problem::EnergyProblem)
     if !energy_problem.solved
         error("The energy_problem has not been solved yet.")
     end
-    save_solution_to_file(output_folder, energy_problem.graph, energy_problem.solution)
+    return save_solution_to_file(output_folder, energy_problem.graph, energy_problem.solution)
 end
 
 """
@@ -507,7 +507,7 @@ The starting value is the value of the previous grouped timesteps or periods or
 The ending value is the value for the grouped timesteps or periods.
 """
 function _interpolate_storage_level!(df, time_column)
-    DataFrames.flatten(
+    return DataFrames.flatten(
         DataFrames.transform(
             df,
             [time_column, :value, :processed_value] =>
diff --git a/src/run-scenario.jl b/src/run-scenario.jl
index c38e59a7..096ee172 100644
--- a/src/run-scenario.jl
+++ b/src/run-scenario.jl
@@ -40,7 +40,7 @@ function run_scenario(
 
     if log_file != ""
         open(log_file, "w") do io
-            show(io, to)
+            return show(io, to)
         end
     end
 
diff --git a/src/tmp.jl b/src/tmp.jl
index 787e93f8..7e0cadb0 100644
--- a/src/tmp.jl
+++ b/src/tmp.jl
@@ -233,7 +233,7 @@ function tmp_create_partition_tables(connection)
         end
         _append_given_durations(appender, row, durations)
     end
-    DuckDB.close(appender)
+    return DuckDB.close(appender)
 end
 
 function tmp_create_union_tables(connection)
@@ -280,7 +280,7 @@ function tmp_create_union_tables(connection)
     )
 
     # Union of all assets, and incoming and outgoing flows
-    DuckDB.execute(
+    return DuckDB.execute(
         connection,
         "CREATE OR REPLACE TEMP TABLE t_union_all AS
         SELECT DISTINCT asset, year, rep_period, time_block_start, time_block_end
@@ -297,7 +297,7 @@ function _append_lowest_helper(appender, group, s, e)
     end
     DuckDB.append(appender, s)
     DuckDB.append(appender, e)
-    DuckDB.end_row(appender)
+    return DuckDB.end_row(appender)
 end
 
 function tmp_create_lowest_resolution_table(connection)
diff --git a/src/variables/investments.jl b/src/variables/investments.jl
index 339bc4a8..3070e3a6 100644
--- a/src/variables/investments.jl
+++ b/src/variables/investments.jl
@@ -17,7 +17,7 @@ function _create_investment_variable!(
             base_name = "$name[" * join(keys_from_row(row), ",") * "]"
         ) for row in eachrow(this_var.indices)
     ]
-    this_var.lookup = OrderedDict(
+    return this_var.lookup = OrderedDict(
         keys_from_row(row) => var for
         (var, row) in zip(this_var.container, eachrow(this_var.indices))
     )
diff --git a/utils/csv-modifications.jl b/utils/csv-modifications.jl
index 654a16c6..647dc4d9 100644
--- a/utils/csv-modifications.jl
+++ b/utils/csv-modifications.jl
@@ -41,9 +41,9 @@ Saves the CSV file at `path`.
 """
 function write_tulipa_csv(tulipa_csv::TulipaCSV, path)
     open(path, "w") do io
-        println(io, join(tulipa_csv.units, ","))
+        return println(io, join(tulipa_csv.units, ","))
     end
-    CSV.write(path, tulipa_csv.csv; append = true, writeheader = true)
+    return CSV.write(path, tulipa_csv.csv; append = true, writeheader = true)
 end
 
 """
@@ -59,7 +59,7 @@ The `content` can be a value or a vector of proper size.
 function add_column(tulipa_csv::TulipaCSV, unit::String, colname, content, position::Int)
     @debug "Adding column $colname ($unit) at $position"
     insert!(tulipa_csv.units, position, unit)
-    insertcols!(tulipa_csv.csv, position, Symbol(colname) => content)
+    return insertcols!(tulipa_csv.csv, position, Symbol(colname) => content)
 end
 
 function add_column(
@@ -69,7 +69,7 @@ function add_column(
     unit = "",
     position = size(tulipa_csv.csv, 2) + 1,
 )
-    add_column(tulipa_csv, unit, colname, content, position)
+    return add_column(tulipa_csv, unit, colname, content, position)
 end
 
 """
@@ -136,7 +136,7 @@ end
 function change_file(f, path)
     tulipa_csv = TulipaCSV(path)
     f(tulipa_csv)
-    write_tulipa_csv(tulipa_csv, path)
+    return write_tulipa_csv(tulipa_csv, path)
 end
 
 input_files_folders = [
diff --git a/utils/scripts/assets-data-revamp-2.jl b/utils/scripts/assets-data-revamp-2.jl
index 05669c3f..95e1a247 100644
--- a/utils/scripts/assets-data-revamp-2.jl
+++ b/utils/scripts/assets-data-revamp-2.jl
@@ -31,7 +31,7 @@ apply_to_files_named("asset-both.csv") do path
 
             # Remove from asset-both
             df = t_both_csv.csv
-            unit, _ = remove_column(t_both_csv, "units_on_cost")
+            return unit, _ = remove_column(t_both_csv, "units_on_cost")
         end
     end
 end
@@ -54,7 +54,7 @@ apply_to_files_named("flow-both.csv") do path
 
             # Remove from flow-both
             df = t_both_csv.csv
-            unit, _ = remove_column(t_both_csv, "variable_cost")
+            return unit, _ = remove_column(t_both_csv, "variable_cost")
         end
     end
 end
diff --git a/utils/scripts/assets-data-revamp.jl b/utils/scripts/assets-data-revamp.jl
index 23b7574b..1437a7ce 100644
--- a/utils/scripts/assets-data-revamp.jl
+++ b/utils/scripts/assets-data-revamp.jl
@@ -79,7 +79,7 @@ apply_to_files_named("asset.csv"; include_missing = true) do path
                 ",
             ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
@@ -115,7 +115,7 @@ apply_to_files_named("flow.csv"; include_missing = true) do path
                 ",
             ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
@@ -157,7 +157,7 @@ apply_to_files_named("asset-milestone.csv"; include_missing = true) do path
                 ",
             ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
@@ -190,7 +190,7 @@ apply_to_files_named("flow-milestone.csv"; include_missing = true) do path
             ",
         ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
@@ -235,7 +235,7 @@ apply_to_files_named("asset-commission.csv"; include_missing = true) do path
                 ",
             ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
@@ -275,7 +275,7 @@ apply_to_files_named("flow-commission.csv"; include_missing = true) do path
                 ",
             ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
@@ -319,7 +319,7 @@ apply_to_files_named("asset-both.csv"; include_missing = true) do path
             ",
         ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
@@ -362,7 +362,7 @@ apply_to_files_named("flow-both.csv"; include_missing = true) do path
             ",
         ) |> DataFrame
 
-        tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
+        return tcsv.units = ["" for _ in 1:size(tcsv.csv, 2)]
     end
 end
 
diff --git a/utils/scripts/create-multiyear-data.jl b/utils/scripts/create-multiyear-data.jl
index c7087560..a9532c2e 100644
--- a/utils/scripts/create-multiyear-data.jl
+++ b/utils/scripts/create-multiyear-data.jl
@@ -31,15 +31,15 @@ cd(multiyear_folder) do
         change_file(filename) do tcsv
             df_2050 = copy(tcsv.csv)
             df_2050.year .= 2050
-            tcsv.csv = [tcsv.csv; df_2050]
+            return tcsv.csv = [tcsv.csv; df_2050]
         end
     end
 
     change_file("assets-data.csv") do tcsv
-        tcsv.csv[end, :active] = "false"
+        return tcsv.csv[end, :active] = "false"
     end
 
     change_file("flows-data.csv") do tcsv
-        tcsv.csv[end, :active] = "false"
+        return tcsv.csv[end, :active] = "false"
     end
 end
diff --git a/utils/scripts/prepare-for-multiyear.jl b/utils/scripts/prepare-for-multiyear.jl
index 143bbfdd..b4cdbbac 100644
--- a/utils/scripts/prepare-for-multiyear.jl
+++ b/utils/scripts/prepare-for-multiyear.jl
@@ -9,7 +9,7 @@ run(`git restore test/inputs/ benchmark/EU/`)
 for filename in ["assets-data.csv", "flows-data.csv"]
     apply_to_files_named(filename) do path
         change_file(path) do tcsv
-            add_column(tcsv, "year", 2030; position = 5)
+            return add_column(tcsv, "year", 2030; position = 5)
         end
     end
 end
@@ -24,7 +24,7 @@ apply_to_files_named("year-data.csv"; include_missing = true) do path
     touch(path)
     change_file(path) do tcsv
         tcsv.units = ["", "h"]
-        tcsv.csv = DataFrame(:year => [2030], :length => [8760])
+        return tcsv.csv = DataFrame(:year => [2030], :length => [8760])
     end
 end
 
@@ -53,7 +53,7 @@ apply_to_files_named("timeframe-data.csv"; include_missing = true) do path
             ),
         )
         tcsv.units = ["", ""]
-        add_column(tcsv, "year", 2030; position = 1)
+        return add_column(tcsv, "year", 2030; position = 1)
     end
 end
 
@@ -69,7 +69,7 @@ for filename in [
         change_file(path) do tcsv
             idx = findfirst(==("rep_period"), names(tcsv.csv))
             @assert idx !== nothing
-            add_column(tcsv, "year", 2030; position = idx)
+            return add_column(tcsv, "year", 2030; position = idx)
         end
     end
 end
@@ -81,7 +81,7 @@ for filename in ["profiles-timeframe.csv", "rep-periods-mapping.csv"]
         change_file(path) do tcsv
             idx = findfirst(==("period"), names(tcsv.csv))
             @assert idx !== nothing
-            add_column(tcsv, "year", 2030; position = idx)
+            return add_column(tcsv, "year", 2030; position = idx)
         end
     end
 end
@@ -99,7 +99,7 @@ for filename in [
         change_file(path) do tcsv
             idx = findfirst(==("profile_type"), names(tcsv.csv))
             @assert idx !== nothing
-            add_column(tcsv, "year", 2030; position = idx)
+            return add_column(tcsv, "year", 2030; position = idx)
         end
     end
 end
@@ -108,13 +108,13 @@ end
 # Maybe this should be renamed to `assets-year-partitions.csv`?
 apply_to_files_named("assets-timeframe-partitions.csv") do path
     change_file(path) do tcsv
-        add_column(tcsv, "year", 2030; position = 2)
+        return add_column(tcsv, "year", 2030; position = 2)
     end
 end
 
 # And don't forget groups
 apply_to_files_named("groups-data.csv") do path
     change_file(path) do tcsv
-        add_column(tcsv, "year", 2030; position = 2)
+        return add_column(tcsv, "year", 2030; position = 2)
     end
 end
diff --git a/utils/scripts/split-assets-year-data.jl b/utils/scripts/split-assets-year-data.jl
index 0f52460c..46c8a930 100644
--- a/utils/scripts/split-assets-year-data.jl
+++ b/utils/scripts/split-assets-year-data.jl
@@ -17,7 +17,7 @@ apply_to_files_named("vintage-assets-data.csv"; include_missing = true) do path
     t_assets_data = TulipaCSV(joinpath(dirname(path), "assets-data.csv"))
     change_file(path) do tcsv
         tcsv.units = ["", "", "kEUR/MW/year", "kEUR/MW/year"]
-        tcsv.csv =
+        return tcsv.csv =
             t_assets_data.csv[:, [:name, :commission_year, :fixed_cost, :investment_cost]] |> unique
     end
 end
@@ -33,7 +33,7 @@ apply_to_files_named("graph-assets-data.csv") do path
             add_column(tcsv, "discount_rate", 0.05)
 
             remove_column(t_assets_data, :capacity)
-            remove_column(t_assets_data, :technical_lifetime)
+            return remove_column(t_assets_data, :technical_lifetime)
         end
     end
 end
@@ -50,7 +50,7 @@ apply_to_files_named("vintage-flows-data.csv"; include_missing = true) do path
     t_assets_data = TulipaCSV(joinpath(dirname(path), "flows-data.csv"))
     change_file(path) do tcsv
         tcsv.units = ["asset_name", "asset_name", "kEUR/MW"]
-        tcsv.csv = t_assets_data.csv[:, [:from_asset, :to_asset, :investment_cost]] |> unique
+        return tcsv.csv = t_assets_data.csv[:, [:from_asset, :to_asset, :investment_cost]] |> unique
     end
 end
 
@@ -63,7 +63,7 @@ apply_to_files_named("graph-flows-data.csv") do path
             append!(tcsv.units, ["MW"])
             leftjoin!(tcsv.csv, df; on = [:from_asset, :to_asset])
 
-            remove_column(t_flows_data, :capacity)
+            return remove_column(t_flows_data, :capacity)
         end
     end
 end
diff --git a/utils/scripts/split-graph-data.jl b/utils/scripts/split-graph-data.jl
index d0fe9a47..280e1f46 100644
--- a/utils/scripts/split-graph-data.jl
+++ b/utils/scripts/split-graph-data.jl
@@ -26,12 +26,12 @@ apply_to_files_named("assets-data.csv") do path
             tcsv.csv = unique(old_tcsv.csv[:, indices])
 
             idx = findfirst(names(tcsv.csv) .== "investment_cost")
-            add_column(tcsv, "investment_method", "simple"; unit = "{none;simple;compact}")
+            return add_column(tcsv, "investment_method", "simple"; unit = "{none;simple;compact}")
         end
 
         indices = setdiff(1:length(old_tcsv.units), columnindex.(Ref(old_tcsv.csv), static_cols))
         old_tcsv.units = old_tcsv.units[indices]
-        old_tcsv.csv = old_tcsv.csv[:, indices]
+        return old_tcsv.csv = old_tcsv.csv[:, indices]
     end
 end
 
@@ -50,17 +50,17 @@ apply_to_files_named("flows-data.csv") do path
             indices = columnindex.(Ref(old_tcsv.csv), cols)
 
             tcsv.units = old_tcsv.units[indices]
-            tcsv.csv = unique(old_tcsv.csv[:, indices])
+            return tcsv.csv = unique(old_tcsv.csv[:, indices])
         end
 
         indices = setdiff(1:length(old_tcsv.units), columnindex.(Ref(old_tcsv.csv), static_cols))
         old_tcsv.units = old_tcsv.units[indices]
-        old_tcsv.csv = old_tcsv.csv[:, indices]
+        return old_tcsv.csv = old_tcsv.csv[:, indices]
     end
 end
 
 apply_to_files_named("year-data.csv") do path
     change_file(path) do tcsv
-        add_column(tcsv, "is_milestone", true; unit = "{true;false}")
+        return add_column(tcsv, "is_milestone", true; unit = "{true;false}")
     end
 end