open configure/Configure

static. =
    DIFF_AVAILABLE = $(CheckProg diff)

if $(not $(DIFF_AVAILABLE))
    Shell. +=
        diff(argv) =
            while $(and $(gt $(length $(argv)), 2), $(mem $(nth 0, $(argv)), -u -c))
                argv = $(nth-tl 1, $(argv))

            if $(not $(eq $(length $(argv)), 2))
                eprintln($"Shell.diff: USAGE:")
                eprintln($"    diff file1 file2")
                exit 2
            src = $(nth 0, $(argv))
            dst = $(nth 1, $(argv))
            if $(not $(test -r $(src)))
                eprintln($"$(src) is not readable")
                exit 2
            if $(not $(test -r $(dst)))
                eprintln($"$(dst) is not readable")
                exit 2
            eof = false
            code = 0
            ch_src = $(fopen $(src), r)
            ch_dst = $(fopen $(dst), r)
            l_dst =
            l_src =
            line = 0
            eof_src = false
            eof_dst = false
            while $(not $(eof))
                line = $(add $(line), 1)
                eof_src = false
                try
                    l_src = $(ch_src.readln)
                    export
                default
                    eof_src = true
                    export
                eof_dst = false
                try
                    l_dst = $(ch_dst.readln)
                    export
                default
                    eof_dst = true
                    export
                if $(not $(and $(equal $(eof_src), $(eof_dst)), $(equal $(l_src), $(l_dst))))
                    eprintln($"Mismatch on line $(line):")
                    eprintln($"- $(src):")
                    eprintln($"  $(if $(eof_src), END OF FILE, $(l_src))")
                    eprintln($"- $(dst):")
                    eprintln($"  $(if $(eof_dst), END OF FILE, $(l_dst))")
                    exit 1
                eof = $(or $(eof_src), $(eof_dst))
            close($(ch_src)) 
            close($(ch_dst)) 
            return true

    export