flan/lib/tast.ml

(** The typed IR: what the checker produces and what every backend consumes.

    Every backend shares this — the LLVM emitter and the hand-written x86-64
    one, each of them under dev redefinition and under the release AOT build
    (plan.org, Compilation) — so everything a backend would otherwise have to
    re-derive is resolved here and nowhere else:

    - names are gone. A local is a slot index into the frame, a global is a
      name, and a call names its callee directly. No environment lookup.
    - field access is an index, not a string, and any auto-deref the source
      relied on is an explicit [Deref] node.
    - literals have a machine type. There is no untyped 1 past this point.
    - a struct literal lists every field in declaration order, with the omitted
      ones filled in as [Zero] — ZII is settled here rather than at runtime.
    - sugar is already gone from the AST; what is left is the small set below. *)

type prim =
  (* arithmetic and comparison, per machine type — the operands carry their own
     kind at runtime, so one constructor covers every width *)
  | Add | Sub | Mul | Div | Rem
  | Eq | Ne | Lt | Le | Gt | Ge
  | Not
  (* bitwise, integers only. [Shr] is arithmetic on a signed type and logical
     on an unsigned one, which is what the operand's own kind already says. *)
  | BitAnd | BitOr | BitXor | Shl | Shr
  (* containers: fixed arrays and slices only at milestone 2 *)
  | Len | At | Slice
  (* (slice-from-ptr p n): a [T] made out of a (Ptr T) and a length the caller
     supplies. It builds the same two words [Slice] builds and allocates
     nothing — the storage stays whoever's it was, which in practice is C's.
     The one thing the compiler cannot check is whether n is the truth; see
     check.ml's "slice-from-ptr" case for what it can. *)
  | SliceFromPtr
  (* the milestone-2 host primitives, plan.org. The four conversions are
     *text*: bytes->f64 parses "12.5", f64->bytes renders it — that is what
     calc-me's tokenizer and the prelude's printers each need. *)
  | Bytes | BytesToF64 | BytesToI64 | F64ToBytes | I64ToBytes
  (* (string b): the other direction of [Bytes], and the same non-instruction.
     See check.ml's "string" case for why it is unchecked. *)
  | StrOfBytes
  (* No surface name: the structural printer is the only thing that builds
     these. U64ToBytes because u64 is not i64 with a flag, EscapeBytes for a
     string nested inside a printed structure. *)
  | U64ToBytes | EscapeBytes
  | WriteStdout | Exit | Argv
  (* A call into the runtime's C, named by symbol. The argument and result
     LLVM types come off the expression nodes themselves, so one constructor
     covers every entry point the allocator and container runtime has and the
     backend grows one arm rather than one per operation — which matters
     because spec-memory.md's runtime is type-erased and therefore *is* a list
     of C entry points. A string or slice argument crosses as ptr+len, the
     same rule as every other shim here. No transfer guard follows one: a
     transfer cannot cross a C frame. *)
  | Rt of string
  (* spec-memory.md, "Alignment": a property of the type, computed at the call
     site, passed as a parameter to the type-erased allocator — all three, and
     they are not alternatives. The checker builds these at the site where the
     concrete element type is known and the backend fills in the number from
     the same layout calculator DWARF uses. *)
  | SizeOf of Types.t
  | AlignOf of Types.t
  (* The address of any expression, not only of a place: the element a [push]
     copies may be a computed value, and the runtime takes it by pointer
     because it is type-erased. The backend already spills a non-place to a
     temporary for exactly this. *)
  | AddrOf
  | Cast of Types.t

type expr = { e : expr_kind; ty : Types.t; loc : Loc.t }

and expr_kind =
  | Int    of int64 * Types.ikind
  | Float  of float * Types.fkind
  | Bool   of bool
  | Str    of string
  | Unit
  | Zero   of Types.t                 (* ZII: all-bytes-zero of this type *)
  | Uninit of Types.t                 (* the explicit opt-out *)
  (* [(filled b)]: every byte of this type set to [b]. [Zero] with a byte the
     program chooses, and a separate node rather than a byte on [Zero] because
     that byte is an *expression* — a memset's operand, not a literal — and
     every reader of [Zero] treats it as a leaf with nothing under it.

     [(dead-beef PATTERN)]: the pattern's four bytes repeating, ascending
     through the storage, so a hex dump reads the pattern left to right —
     DEADBEEF by default, and whatever u32 was written otherwise. The operand
     is always present by the time it reaches here: [(dead-beef)] is checked
     into [(dead-beef 0xDEADBEEF)], so a backend has one shape to lower and
     the default cannot acquire a code path of its own. A size that is not a
     multiple of four ends on a prefix of the pattern — DE, DE AD, DE AD BE
     for the default — which both backends produce identically.

     The operand is an expression and not an [int32] for the same reason
     [Fill]'s byte is: it may be computed. A literal one is folded by each
     backend into the immediate it always was; anything else is evaluated and
     byte-reversed at run time. *)
  | Fill     of Types.t * expr
  | DeadBeef of Types.t * expr
  | Local  of int                     (* slot index into the frame *)
  | Global of string
  | Prim   of prim * expr list
  | Call   of string * expr list      (* a call naming its callee *)
  (* The address of a function the compiler emitted, by symbol. Which symbol
     table, and whether the surface language can see it, is [fnref]'s job.

     Two unrelated consumers, and the difference between them is the whole
     reason [fnref] has three cases rather than two. The compiler's own uses —
     the Map's hash and equality pair (Odin's [Map_Info] is two contextless
     [proc] fields reached exactly this way) and a handler-bind clause's
     symbol — want the *symbol*, always, and carry the Flan type [Alloc]. A
     function *value* someone wrote wants the body that is current, which in a
     dev build is not the symbol but whatever the indirection cell holds, and
     carries the Flan type [Fn]. *)
  | FnAddr of fnref
  (* A function value with an environment: the lifted body, and the address of
     the copies the enclosing frame is holding for it. The environment is a
     [Make] of a struct the checker synthesised, stored into a slot of the
     frame the literal was written in, so this node's second half is an
     [Addr (Plocal _)] and the copies were taken where the value was made.

     Its own node rather than a field on [FnAddr] because the two answer
     different questions: [FnAddr] is an address, and is asked for by three
     unrelated readers that want a bare symbol ([Alloc]-typed, see [fnref]),
     while this is a *value* of type [Fn] and can never be anything else.

     What stops it dangling is the checker, not this node: a value carrying an
     environment may not leave the frame that owns it, so every position that
     would outlive the frame is refused. spec-memory.md's case 2, and the
     escaping half — an environment the collector allocates — is the case the
     refusals name. *)
  | Closure of fnref * expr
  (* A (CFn ...) value where a (Fn ...) is wanted. The one coercion between
     the two function types, and it goes this way only: there is nowhere for
     an environment to go in the other direction.

     The pair it builds is {thunk, the address}: the *thunk's* code, one per
     signature, with the original bare address stored where an environment
     would be. The thunk reads it back out and calls it. So a value reached
     through this is reached by a body that really does take an environment,
     which is what keeps every indirect call exactly typed — including on
     wasm32, where [call_indirect] checks the signature and an argument the
     callee did not declare is a trap rather than a register nobody reads.

     The string is the thunk's name, minted and memoised by the checker: the
     backends emit the pair and derive nothing. What it costs is one hop per
     call, paid by a *name* handed to an [Fn]-typed parameter and by nothing
     else — a literal, capturing or not, is compiled to take an environment
     and needs no thunk. A signature that wants the address alone writes
     [CFn] and pays nothing at all, which is what the type is for. *)
  | Thicken of string * expr
  (* A call through a function value: the callee is an expression of type
     [Fn], not a name. Its own node rather than a [Call] with an expression in
     the name slot, because everything that walks this IR treats [Call]'s
     string as a *link-time* edge — [Reach] roots the callee, [Dev] finds the
     cell to redefine, [Emit] may load that cell — and none of those are
     questions an indirect call can answer. Keeping them apart means each of
     those readers keeps working on the direct case unchanged and says
     explicitly what it does with the indirect one. *)
  | CallPtr of expr * expr list
  | Do     of expr list
  | Let    of (int * expr) list * expr list
  | If     of expr * expr * expr
  (* condition, body, and the *latch*: forms that run after the body and before
     the condition is tested again. [dotimes] folds its increment in there
     rather than onto the end of the body, because a [continue] branches to the
     latch and a step written in the body would be skipped — the loop would
     never advance and would hang. A [while] has an empty latch. *)
  | While  of expr * expr list * expr list
  | Return of expr option
  (* Leaving a loop, and jumping to its latch. The int is how many loops out
     the target is, innermost first: 0 is the loop this is directly inside.
     A *relative* depth rather than a name or an id because it is exactly what
     each backend already has — [emit] keeps one entry per [While] it is inside
     and indexes it. The invariant that makes it sound: the checker mints these
     only from its own loop stack, and both stacks are pushed once per [While].
     A [While] the checker *invents* (alloc_guard, the file-failure retry) is
     built directly and never contains one of these, so the entry it pushes in
     [emit] matches nothing and is harmless — keep it that way.

     [check_loop]'s [While] is the one exception and the exception proves the
     rule: it *is* pushed on [ctx.loops], so the [Break 0] that leaves it and
     every [Continue] a [recur] mints are counted against the same stack [emit]
     indexes. Invented is not the property that matters; being on the stack
     is. *)
  | Break    of int
  | Continue of int
  | Set    of place * expr
  | Field  of expr * int              (* target is already a struct value *)
  | Addr   of place
  | Deref  of expr
  | Make   of string * expr list      (* struct literal, every field, in order *)
  (* A data type value: the data type's name, the case's name, and every
     field of that case in declaration order with the omitted ones filled in
     as [Zero] — the
     same ZII rule [Make] carries, and settled here for the same reason. It is
     its own node rather than a [Make] over a synthesised struct because the
     value's *type* is the data type and its payload is a byte blob the case is
     reinterpreted into; a backend that saw only [Make] would have to rederive
     which of the two it was looking at. *)
  | MakeCase of string * string * expr list
  (* One field of one case of a data type value, by index. The case name is on the
     node because the payload is untyped bytes: [Field]'s index alone cannot
     say which case struct the blob is being read as. [match] is the only thing
     that proves the case, so this is only ever built under an arm that
     checked the tag — and by [Render], which reads a field only after the same
     comparison. One node, so the payload layout is known in exactly one place
     in each backend rather than once per reader. *)
  | CaseField of expr * string * int
  | Arr    of expr list               (* fixed-array literal *)
  | Some_  of expr
  | None_
  | Match  of expr * arm list
  (* (some x): unwrap Some, else early-return None from the enclosing function.
     An early return, not an expression that can fail — hence its own node. *)
  | UnwrapSome of expr
  (* Conditions, spec-conditions.md. [Signal] walks the handler stack and
     returns Unit whatever it finds — with nothing matching it is a no-op, so
     nothing here alters control flow. [HandlerBind] pushes one frame per
     clause, runs its body, and pops them; each clause was lifted into its own
     function by the checker, so what is left is the frame and the call. *)
  | Signal  of sigkind * int * expr   (* how, the type id, the condition *)
  | Handled of hframe list * expr list
  (* The transfer, spec-conditions.md §3–§6. [RestartCase] pushes one frame per
     clause, runs its body, and pops them; if a transfer arrives naming one of
     *its* frames it runs that clause instead, and the whole form yields either
     way. [InvokeRestart] looks the name up on the restart stack, writes the
     frame it found into the transfer channel and leaves — it has type Never,
     so nothing follows it.

     [InvokeRestart]'s arguments are already evaluated: the checker binds each
     to a slot and wraps the node in a [Let], so what is left here is a list of
     locals to copy into the frame. Two reasons, and both matter. An argument
     that transfers on its own must be guarded before this one aims the
     channel; and a call written in an argument has to be on the walk [Reach]
     and [Load] already do, which a list hanging off a node they treat as a
     leaf would not be. [rsig] is the argument types as written, and [rsig_id]
     their hash — §3's run-time check, since the name is resolved on a stack
     nothing static can see.

     Not every one of these was written as a [restart-case]. A [handler-case]
     is a [Handled] whose clause invokes a restart, wrapped in one of these to
     catch it — see [Check.check_handler_case] — so the unwinding handler
     reaches a backend as this node and needs nothing of its own. A clause
     whose name begins with [handler-case/] is one the checker made up for
     that. *)
  | RestartCase of rclause list * expr
  (* (with-allocator A BODY...) — spec-memory.md. It rebinds the current
     allocator for its dynamic extent and releases nothing. Its own node
     because the restore has to happen on the *transfer* path too: a body that
     errors, or a restart taken from inside it, must not leave the context
     allocator pointing at a region the handler knows nothing about. *)
  | WithAlloc of expr * expr list
  (* name id, name, arguments, their spelling, its hash, where *)
  | InvokeRestart of int * string * expr list * string * int * Loc.t

(* [Serror] is §2's diverging variant: the same lookup, type Never, and with
   nothing transferring the program stops rather than carrying on. *)
(* Which symbol table the address comes out of. [Flanfn] is a function this
   compiler emitted and is therefore name-mangled and reachability-tracked;
   [Rtfn] is a C entry point in flan_rt.c, spelled as written. The two are
   interchangeable at the call site because a Flan function's emitted signature
   is its parameters followed by the transfer channel, and the runtime's
   matching typedef spells that last pointer out. *)
(* [Flanfn] is a function this compiler emitted, named by its mangled symbol,
   and always the symbol itself. [Rtfn] is a C entry point in flan_rt.c, spelled
   as written. [Fnval] is also a Flan function this compiler emitted, but as a
   *value* someone asked for by writing its name — and it is a separate case
   because a dev build must answer it with the current body rather than with the
   original symbol, which means a load from the indirection cell. The first two
   must never take that path: a lifted handler clause and a hash pair have no
   cell to load from. The three are interchangeable at a call site, because a
   Flan function's emitted signature is its parameters followed by the transfer
   channel and the runtime's matching typedef spells that last pointer out. *)
and fnref = Flanfn of string | Rtfn of string | Fnval of string

and sigkind = Ssignal | Serror

and place =
  | Plocal  of int
  | Pglobal of string
  | Pfield  of expr * int
  | Pindex  of expr * expr list
  | Pderef  of expr

(* A pushed handler: which condition type it matches, the lifted function that
   runs when one is signalled, and the environment that function is handed.

   [henv] is the address of the establishing frame's copies of whatever the
   clause captured, or [None] when it captured nothing. It is sound for the
   same reason the frame itself is: a handler frame is popped by the body that
   pushed it, so it can never be reached from outside the extent of the
   function whose stack both it and the environment live on. There is no
   escaping case here to defer. *)
and hframe = { htype : int; hfn : string; henv : expr option }

(* A restart clause. [rname_id] is what [invoke-restart] matches by name; the
   body is a branch in the function that wrote it, because unlike a handler a
   clause runs at the restart-case, which is where it was written.

   [rparams] are the slots §3's parameters are bound to, in order, with their
   types; the invoker stores into a buffer this frame owns and the clause loads
   them from it. [rsig] is how those types are spelled and [rsig_id] its hash:
   what the two ends compare, since neither can see the other. *)
and rclause =
  { rname_id : int; rname : string; rparams : (int * Types.t) list;
    rsig : string; rsig_id : int; rbody : expr list }

(* [binds] are the slots the pattern's fields are bound to, in field order. *)
and arm = { acase : string option; binds : int list; abody : expr list }

type field = { fname : string; fty : Types.t }

type structure = { sname : string; fields : field list }

type variant = { vname : string; vfields : field list }

type data = { dname : string; cases : variant list }

type fn = {
  name    : string;
  params  : Types.t list;             (* bound to slots 0 .. n-1, in order *)
  slots   : Types.t array;            (* the frame: one entry per slot *)
  (* What the source called each slot, parallel to [slots]. [None] is a slot
     the compiler made up and no one wrote a name for -- [dotimes]'s hidden
     bound, the pair (min) and (max) evaluate their operands into, the slot a
     tail expression goes through. Names are otherwise gone from this IR (see
     the header); this is the one exception, and it exists so a debug build can
     emit a [!DILocalVariable] that says [lo] where the source said [lo]. A
     backend is free to ignore it entirely -- nothing is *resolved* through it,
     and a slot is still only ever referred to by index. *)
  snames  : string option array;
  ret     : Types.t;
  body    : expr list;
  (* The defers again, innermost first. [body] already has them spliced onto
     the normal exit path; this is the same list for the *transfer* exit path,
     which leaves through a landing block the backend builds and no form in
     [body] can reach. spec-conditions.md §5: they run, and errdefer does not.

     Not quite the same list: each one here is wrapped in a test of the
     counter [Check.register_defer] stores into, because a transfer can start
     above a defer the text has not reached — in the initialiser of the very
     [let] whose body it is written in, which is [slurp]'s shape — and a
     cleanup over a binding nothing wrote is not cleanup. The normal paths
     need no test: falling off the end is below every defer, and a [return]
     carries only the ones above it. A backend runs this list as it stands. *)
  fdefers : expr list;
  (* Set on a function the checker made up rather than one anyone wrote: a
     handler-bind clause, lifted out of the function named here. It is reached
     by address from that function's body and from nowhere else, so it needs no
     cell and no registry slot, and a redefinition of the parent carries its
     own copy. *)
  fparent : string option;
  (* The slot the environment parameter is stored into, on a function that
     was lifted out of something and captures one of its locals. Every
     emitted signature takes the environment (see Emit's [env_param]) and
     almost every function ignores it; this is the one that does not, and it
     says where the pointer goes rather than fixing an index by convention,
     because the slot is minted by [fresh_slot] like any other and a rule of
     the form "the slot after the parameters" would be a second thing to keep
     in step with the allocation order.

     [None] on everything anyone wrote. A capturing body reads its copies out
     of this pointer once, at entry, into named slots of its own — so the
     copy the value was made with is the copy the body sees. *)
  fenv    : int option;
  floc    : Loc.t;
}

(* [gfolded] is the difference between a constant whose value the *checker*
   consumed — an array length, decided before any type resolves — and one that
   is only ever read at run time. The first is in the program's shape and can
   never be reloaded; the second is just bytes in memory and can. Nothing else
   can tell them apart afterwards, so it is recorded here. *)
type global = {
  gname   : string;
  gty     : Types.t;
  ginit   : expr;
  gconst  : bool;
  gfolded : bool;
  (* [def] rather than [defonce]: the initialiser runs on every daemon
     re-run instead of once behind a flag. [Emit.startup_plan] is the
     consumer of that. [Session.eval] is the other reader, for the half of
     [defparameter] that is not about re-runs at all — evaluating one
     assigns, so a re-evaluated [def] stores its new value at the next frame
     boundary as well. *)
  grerun  : bool;
}

(* A foreign function: no body, and [esym] is the symbol the linker sees. The
   aggregate calling convention is not modelled here — a C shim flattens every
   struct that crosses the boundary, so clang classifies it per target and
   nothing in the backend has to know x86-64 from arm64 from wasm32. *)
type extern = {
  ename   : string;                   (* the Flan name, e.g. rl/init-window *)
  esym    : string;                   (* the C symbol *)
  (* Where the [declare] was written. Carried so a refusal over a foreign
     signature can point at it rather than at <unknown>, which is what the
     dyn boundary check had to do before this field existed. *)
  eloc    : Loc.t;
  eparams : Types.t list;
  eret    : Types.t;
}

type program = {
  structs : structure list;
  datas  : data list;
  (* The untagged unions, carried as [structure] values: a union's members are
     a field list whose every offset is zero, so the record a struct uses says
     all of it. Which list a name came out of is what a backend reads to know
     whether to accumulate the offsets or not. *)
  unions : structure list;
  globals : global list;              (* in declaration order *)
  externs : extern list;
  fns     : fn list;
  (* The C the program's own (declare-c ...) forms generated, if any: one
     translation unit, compiled into the build like a package's hand-written
     .c file. It is on the program rather than beside it so that every driver
     — the CLI, the REPL, the acceptance table — carries it without knowing
     it exists. See [Shim]. *)
  (* The generated FFI shim, in parts keyed by the declaration each serves,
     with "" for the shared preamble. Parts rather than one string so that
     [Reach.link] can drop a wrapper whose binding nothing reachable calls. *)
  cshim   : (string * string) list;
}

(* ── Walking an expression ─────────────────────────────────────────── *)

(* Every node of an expression, outermost first, the ones hanging off a [place]
   included. One traversal in the IR's own file rather than one per reader:
   three passes ask structural questions of a body — what names it refers to
   ([Reach]), whether a global's initialiser can transfer, and which globals it
   reads ([Check]) — and each of them that spelled the traversal out again was
   a place a new constructor could be forgotten in. What differs between those
   readers is the question, which is [f]. The shape of the IR is not theirs to
   restate.

   A lifted clause's body is not in here: it is a function of its own, and this
   walks one expression. A reader that wants it follows [hfn], the way [Reach]
   does. *)
let rec walk (f : expr -> unit) (e : expr) =
  f e;
  let go = walk f in
  let gos = List.iter go in
  match e.e with
  | Int _ | Float _ | Bool _ | Str _ | Unit | Zero _ | Uninit _ | Local _
  | Global _ | None_ | FnAddr _ | Break _ | Continue _ -> ()
  (* Both carry an operand, and it must be walked: a call written inside a
     fill's byte or a dead-beef's pattern is a call, and [Reach] roots what it
     finds here. A leaf row would drop it silently — the match would still
     compile. *)
  | Fill (_, b) | DeadBeef (_, b) -> go b
  | Prim (_, es) | Call (_, es) | Do es | Make (_, es) | MakeCase (_, _, es)
  | Arr es | InvokeRestart (_, _, es, _, _, _) -> gos es
  | CallPtr (c, es) -> go c; gos es
  | Let (bs, body) -> List.iter (fun (_, v) -> go v) bs; gos body
  | If (a, b, c) -> go a; go b; go c
  | While (c, body, latch) -> go c; gos body; gos latch
  | Return v -> Option.iter go v
  | Set (p, v) -> walk_place f p; go v
  | Addr p -> walk_place f p
  | Field (t, _) | Deref t | CaseField (t, _, _) | Some_ t | UnwrapSome t
  | Signal (_, _, t) | Closure (_, t) | Thicken (_, t) -> go t
  | Match (sc, arms) -> go sc; List.iter (fun a -> gos a.abody) arms
  | Handled (hs, body) ->
    List.iter (fun h -> Option.iter go h.henv) hs; gos body
  | RestartCase (cs, body) -> List.iter (fun c -> gos c.rbody) cs; go body
  | WithAlloc (a, body) -> go a; gos body

and walk_place f (p : place) =
  match p with
  | Plocal _ | Pglobal _ -> ()
  | Pfield (t, _) | Pderef t -> walk f t
  | Pindex (t, idx) -> walk f t; List.iter (walk f) idx

(* ── What the object image can hold ─────────────────────────────────── *)

(* Whether an initialiser is a value a linker can write into the program's
   image: a literal, a zero, an aggregate of those. It is [Emit.const]'s
   accepted set asked as a question rather than answered as a string, and the
   two have to stay the same set — [const] spells the value, this decides who
   is allowed to ask it to.

   Everything else is *computed*, which used to be the end of the road and is
   now a fork: [Check] lifts a computed initialiser into a function of its own
   and the program calls it at startup. So this is no longer "what a global may
   be", only "what needs no code" — which is why a [MakeCase] is false here
   rather than an error. A data type case written into the image would need a
   byte-level encoder that could not encode a string field at all; written as a
   store at startup it needs nothing. *)
(* The pattern [(dead-beef)] means, and the name the builtin is spelled after.
   It lives here rather than in a backend because it is a fact about the
   language — what the bare form of a builtin means — and because the checker
   is what writes it in: a [DeadBeef] node always carries its pattern, so no
   emitter has a default of its own to keep in step with this one. *)
let dead_beef_default = 0xDEADBEEFl

let rec const_init (e : expr) =
  match e.e with
  | Int _ | Float _ | Bool _ | Str _ | Unit | Zero _ | Uninit _ | None_ -> true
  | Make (_, es) | Arr es -> List.for_all const_init es
  | Some_ v -> const_init v
  | _ -> false

(* The declared position of a case, which is its tag, and the case itself. Tags
   are declaration order from zero, so an all-bytes-zero data type is the first
   case with a zeroed payload — the same rule that makes an [Option]'s zero a
   [None], and the reason case order is part of a data type's contract. *)
let case_index (u : data) name =
  let rec go i = function
    | [] -> None
    | (c : variant) :: rest ->
      if String.equal c.vname name then Some (i, c) else go (i + 1) rest
  in
  go 0 u.cases

let vfield_index (c : variant) name =
  let rec go i = function
    | [] -> None
    | (f : field) :: rest ->
      if String.equal f.fname name then Some i else go (i + 1) rest
  in
  go 0 c.vfields

let field_index (s : structure) name =
  let rec go i = function
    | [] -> None
    | f :: rest -> if String.equal f.fname name then Some i else go (i + 1) rest
  in
  go 0 s.fields