Read R code from a file (i.e. source) in a shiny App

I want to modularise my shiny app code, by moving parts of the code in separate files. I then include the content of the file with a call to the source function: source("./www/some_code.R", local = TRUE)

It works well except for an undesired effect: the word TRUE is added just below the insert.

Could you help me understand why this happen and how I can remove this undesired text?

For a reproducible example,

create app.R:

#
# Find out more about building applications with Shiny here:
#
#    http://shiny.rstudio.com/
#


library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

   # Application title
   titlePanel("Old Faithful Geyser Data"),

   # Sidebar with a slider input for number of bins 
   sidebarLayout(

      sidebarPanel(
         source("./www/slider.R", local = TRUE)
      ),

      # Show a plot of the generated distribution
      mainPanel(
         plotOutput("distPlot")
      )
   )
)


# Define server logic required to draw a histogram
server <- function(input, output) {

   output$distPlot <- renderPlot({
      # generate bins based on input$bins from ui.R
      x    <- faithful[, 2] 
      bins <- seq(min(x), max(x), length.out = input$bins + 1)

      # draw the histogram with the specified number of bins

      hist(x, breaks = bins, col = 'darkgray', border = 'white')
   })
}

# Run the application 
shinyApp(ui = ui, server = server)

and in the www folder the slider.R:

sliderInput("bins",
            "Number of bins:",
            min = 1,
            max = 50,
            value = 30)