I’m so sorry to mussy up this section with some non-Haskell blather but today I found a use for python. It has long been a problem of mine that printed documents do not fit comfortably on the bookshelf, and I wanted a method to do this. Maybe if I had googled it better I’d have found something. Here’s some pseudocode.

> type PDF = [Page]                            #[1,2,3,4,5,6,7,8]

> (pdfa, pdfb) = pdf.split(pdf.length() // 2)  #([1,2,3,4], [5,6,7,8]
> pdfc = pdfa.reverse()                        #[4,3,2,1]
> weave(pdfc, pdfb)                            #[4,5,6,3,2,7,8,1]

> weave :: PDF -> PDF -> PDF
> weave [] []         = []
> weave (a:as) (b:bs) = a:b:(weave bs as)

If we did this to a pdf, you could fold the pages together nicely and make a book. It turns out you can pull this off quick in python. Flip on short edge.

    import pypdf
    pdf_in = open('aakj.pdf', 'rb')
    pdf_reader = pypdf.PdfReader(pdf_in)
    b = (- len(pdf_reader.pages)) % 4
    mid = (len(pdf_reader.pages)-b) // 2
    first_half_pdf  = reversed(pdf_reader.pages[:(mid+b)]) 
    second_half_pdf = pdf_reader.pages[(mid+b):]
    i = 1
    pdf_writer = pypdf.PdfWriter()
    # Reverse the first_half_pdf by iterating in reverse order and adding to pdf_writer
    for pagea,pageb in zip(first_half_pdf,second_half_pdf):
        if i%2:
            pdf_writer.add_page(pagea)
            pdf_writer.add_page(pageb)
        else: 
            pdf_writer.add_page(pageb)
            pdf_writer.add_page(pagea)
        i += 1
    i = b
    for c in range(b):
        if i%2:
            pdf_writer.add_blank_page()
            pdf_writer.add_page(pdf_reader.pages[b-c-1])
        else:
            pdf_writer.add_page(pdf_reader.pages[b-c-1])
            pdf_writer.add_blank_page()
        i += 1
    pdf_out = open('aakj (1).pdf', 'wb')
    pdf_writer.write(pdf_out)
    pdf_out.close()
    pdf_in.close()