How to write a basic org-babel-execute function for a new language?

Question

What I'm trying to achieve is very simple: take a source block, write it to a temp file, pass the file name as an argument to an executable which will run it, and then include the output in the #RESULTS block. The snippet below seems close, but when I try to execute the code block the executable (nim) complains that it expected a file name so I guess it passes the block body via stdin instead of temp file:

(defun org-babel-execute:nim (body params)
 "Execute a block of Nim code with org-babel."
 (message "executing Nim source code block")
    (org-babel-eval "nim r " body))

Second attempt, based on @NickD's comment and org-babel-execute:dot:

  (defun org-babel-execute:nim (body params)
    "Execute a block of Nim code with org-babel."
    (message "executing Nim source code block")
    (let* (in-file (org-babel-temp-file "nim-")))
    (with-temp-file in-file
      (insert body))
    (org-babel-eval (format "nim r %s" (org-babel-process-file-name in-file))))

When trying to execute a code block, getting "Symbol's value as variable is void: in-file"

Answer 1

You need to define org-babel-execute:nim:

(defun org-babel-execute:nim (body params)
  "Execute a block of Nim code with org-babel."

While we could use nim via STDIN, it will clutter the current working directory. Therefore, let us use a temporary file instead. nim also needs files to start with a letter and end with .nim, so we adjust the arguments to org-babel-temp-file. For some less noise, let's also change the default verbosity, but make it adjustable:

(let ((in-file (org-babel-temp-file "n" ".nim"))
      (verbosity (or (cdr (assq :verbosity params)) 0)))
  (with-temp-file in-file
    (insert body))

Last but not least, we call org-babel-eval. Since we supplied the original body in our in-file, we can keep the body argument of org-babel-eval empty. One could probably also change the working directory to a temporary directory instead, but I'm not familiar with nim, at all.

(org-babel-eval
 (format "nim compile --verbosity=%d --run %s" verbosity
         (org-babel-process-file-name in-file))
 "")))

Here's all at once:

(defun org-babel-execute:nim (body params)
  "Execute a block of Nim code with org-babel."
  (let ((in-file (org-babel-temp-file "n" ".nim"))
        (verbosity (or (cdr (assq :verbosity params)) 0)))
    (with-temp-file in-file
      (insert body))
    (org-babel-eval
     (format "nim compile --verbosity=%d --run %s" verbosity
             (org-babel-process-file-name in-file))
     "")))

And here's an example:

#+begin_src nim
var name : string = "Woof"
echo "Hi, ", name, "!"
#+end_src

#+RESULTS:
: Hi, Woof!