# Transpiler from J to Fortran > Source: > Published: 2026-08-15 16:09:52+00:00 [J](https://www.jsoftware.com/#/) is an offshoot of APL, co-authored by Iverson, that uses the ASCII character set. Since modern Fortran is also an array programming language, and since it [has been argued](https://martinalderson.com/posts/which-programming-languages-are-most-token-efficient/) that token-efficient languages such as J are a good fit for LLMs, I wrote a transpiler from J to Fortran using OpenAI Sol primarily Besides a batch translator there is a REPL that translates and runs Fortran code on the fly. It works on some small J programs, but it cannot handle the more dynamic programs at [GitHub - jonghough/jlearn: Machine Learning Library, written in J ยท GitHub](https://github.com/jonghough/jlearn) or the programs for astronomy [here](https://pages.astro.umd.edu/~jpha/J_page.html). Claude Opus says that a larger subset of J code could be transpiled to Python/Numpy calling Fortran as needed.