{require lib_BN}

{def genpi
 {def genpi.loop
  {lambda {:n :pi :q :r :t :i :z}
   {if {> :z :n}
    then :pi
    else {let { {:n :n} {:pi :pi} {:q :q} {:r :r} {:t :t} {:i :i} {:z :z}
                {:digit {BN./ {BN.+ {BN.* {BN.- {BN.* :i 27} 12} :q}
                                    {BN.* :r 5} }
                              {BN.* :t 5} } }
                {:u {BN.* {BN.+ {BN.* :i 3} 1}
                          {BN.* 3 {BN.+ {BN.* :i 3} 2} } } }
              } {genpi.loop :n
                            {BN.+ :pi :digit}
                            {BN.* {BN.* :q 1}
                                  {BN.* :i {BN.- {BN.* :i 2} 1} }}
                            {BN.* {BN.* :u 1}
                                  {BN.+ {BN.* :q {BN.- {BN.* :i 5} 2} }
                                        {BN.- :r {BN.* :t :digit} }}}
                            {BN.* :t :u}
                            {BN.+ :i 1}
                            {+ :z 1}} }}}}
 {lambda {:n}
  {genpi.loop :n # 1 180 60 2 0} }}
-> genpi

We can generate π with 72 digits in about 500ms.

{BN.DEC 72}
->  72 digits
{genpi 60}
-> 3.141592653589793238462643383279502884197169399375105820974944592307816406