The prelude is lib/prelude.fln, embedded at build time and read by the indented reader.

TODO.org

@@ -25,6 +25,7 @@ prelude and vendor rewritten in .fln; tests and examples converted with =flan co
 then =flan convert=, the .flan source path and =flan-mode= removed. Nothing tracks what
 .flan can no longer say. Step 1 is done: the name rule with the =is-=/=has-= renames
 through the prelude, vendor and raylib, and =T?=, =??=, =x!=, =if let g = x= and =a?.b=.
+Step 2's prelude is done: =lib/prelude.fln=, read by the indented reader.
 ** DONE x? tests presence, and narrows
 CLOSED: [2026-09-26]
 Decided (133): =x?= is a bool; =if x?=, =elif x?=, =while x?= and the rest of an =and=

docs/BUILT.md

@@ -7527,7 +7527,7 @@ prints freed memory.
 
 ## String is a prelude struct, and the checker is its wall
 
-`String` is `(defstruct String [bytes (Vec u8)])` in `lib/prelude.ml`, so its
+`String` is `struct String(bytes: Vec(u8))` in `lib/prelude.fln`, so its
 allocator, free, retry on exhaustion and dev-registry notes are the Vec's and
 neither backend has a String of its own. What makes it always valid UTF-8 is
 `lib/check.ml`'s String section: outside the prelude the field and the

emacs/flan-cnr.el

@@ -166,7 +166,7 @@ often `pause' itself, and `P' shows them.")
 
 (defconst flan-cnr-breakpoint "Pause"
   "The condition `(pause)' signals.
-`lib/prelude.ml' writes it as an ordinary struct under a `restart-case', so
+`lib/prelude.fln' writes it as an ordinary struct under a `restart-case', so
 nothing in the compiler knows a breakpoint from an error and this buffer is the
 first place that can tell the difference.  Named here rather than spelled at
 its use, because it is a fact about the prelude.")

emacs/flan.el

@@ -1528,7 +1528,7 @@ definition, and RET on a frame or on the stop in the break buffer."
       (user-error "flan: the daemon gave %s an unreadable location: %s"
                   subject loc))
      ((string-match-p "\\`<.*>\\'" (nth 0 parts))
-      ;; The prelude is a string inside the compiler (lib/prelude.ml) and
+      ;; The prelude is a string inside the compiler (lib/prelude.fln) and
       ;; names itself <prelude>; anything in angle brackets is a placeholder
       ;; the frontend made up, not a path.
       (user-error "flan: %s is defined in %s, which is not a file on disk"

lib/body_macros.ml

@@ -179,7 +179,7 @@ let add (t : t) ~qualify forms =
     adds nothing. *)
 let table ?file (forms : Form.t list) : t =
   let t = create () in
-  let prelude = try Reader.read_all ~file:Prelude.file Prelude.source with _ -> [] in
+  let prelude = try Prelude.read () with _ -> [] in
   add t ~qualify:Fun.id prelude;
   (match file with
    | None -> ()

lib/dune

@@ -64,3 +64,17 @@
     (echo "|c}\n\nlet agent_source = {c|\n")
     (cat %{workspace_root}/vendor/agent/flan_agent.c)
     (echo "|c}\n")))))
+
+; The prelude, carried the same way: lib/prelude.fln is the source, read by
+; the indented reader at every build (lib/prelude.ml). No newline after the
+; opening quote, so a location's line is the file's line.
+(rule
+ (target prelude_src.ml)
+ (deps prelude.fln)
+ (action
+  (with-stdout-to
+   prelude_src.ml
+   (progn
+    (echo "let source = {fln|")
+    (cat prelude.fln)
+    (echo "|fln}\n")))))

lib/source.ml

@@ -3,7 +3,8 @@
     ([Reader]). Both give the same [Form.t], so nothing past this point knows
     which one a file was written in, and a program may mix them freely.
 
-    Only program sources come through here. The prelude, the wire protocol and
+    Only program sources come through here. The prelude is indented text
+    embedded in the compiler and read by [Prelude.read]; the wire protocol and
     the registry's spellings are paren text the compiler writes itself, and
     read it with [Reader] directly. *)