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Frepl (Fortran REPL) is an experimental ruby-based REPL for Fortran, that I wrote because I was trying to learn Fortran, and I find the feedback loop you get with a REPL makes learning a language much easier and more enjoyable.

You don't need to know ruby to use Frepl, but you do need to have ruby (at least version 2) installed.

Frepl differs from some other Fortran REPLs (e.g. Fytran) in that rather than typing a whole program then running a separate compile command, Frepl compiles and runs your code each time you hit ENTER, and generally tries to treat your input as an expression, and echo the result of evaluating it, like many REPLs for dynamic languages.

The goal of this design is to provide instant feedback and also permit the user to quickly alter/redefine code they've just entered.

There are a lot of deficiencies with Frepl, namely:

  • Only knows how to classify/parse a limited set of Fortran.
  • IO is fairly hampered. Reading from and writing to a file is mostly supported, but reading from STDIN is currently not supported. I have some ideas about how to sort of accomplish this latter goal, but they are half-baked and convoluted. Also I'm not sure this is even really that important.
  • Parsing is pretty dumb. It uses complicated and somewhat opaque regexes in places where a real lexer/parser approach might be more appropriate, though might also be overkill.

Project plans

See issue tracker.

Known issues

  • Frepl is only able to correctly classify a subset of legal Fortran. At the same time, Frepl will also happily accept some illegal Fortran, and wait for the compiler to tell you about the problem. The UX here is not great.
  • When specifying parameter and dimension in a declaration, parameter must currently come first, e.g. integer, parameter, dimension(:) :: a.
  • only do... end do loops are supported
  • no support for labels; hence, no GOTO and its ilk


$ gem install frepl

It doesn't really make sense to install Frepl as part of an application via the Gemfile, but you could do that if you wanted.


You should be able to run frepl from the command line after having installed the gem.

Alternatively, if you have the source code, you can run rake console from the gem folder.

You will get a prompt, and you can just start typing Fortran, type q to quit.

> integer :: a = 1
> integer, dimension(:), allocatable :: b
> allocate(b(0:4))
> b = [1,2,3,4]
> write(*,*) a * b
           1           2           3           4
> q

Redefine a function, change the type of a variable, etc.

> integer :: a = 1
> a = 3
> real a
> a = 5
> integer function sum(a, b)
>   integer, intent(in) :: a, b
>   sum = a + b
>   end function
> write(*,*) sum(1,2)
> real function sum(a, b, c)
>   real, intent(in) :: a, b, c
>   sum = a + b + c
>   end function
> write(*,*) sum(1.0,2.9,3.4)

Undo the last statement

Type f:z (z as in cmd+z for undo). Handy when you made an error, e.g.

> integer :: a =3
> real :: b
> b = 'fo'

b = 'fo'
Error: Can't convert CHARACTER(1) to REAL(4) at (1)

> f:z
> b = 3.4

You can see some repl commands by typing f:help. Not much going on there, currently.


  1. Fork it ( )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request


Experimental ruby-based Fortran REPL




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