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fix: O(n) nested match decoder replaces O(n²) tuple pattern matching (#111)
* test: add reproduction for exponential compile time bug Large records with 26+ optional fields cause exponential compile time growth due to nested switch statements in generated decoder code. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com> * fix: O(n) nested match decoder replaces O(n²) tuple pattern matching The previous decoder generated O(n²) pattern matches for records with n fields, causing exponential compile time growth in the ReScript type checker. This fix uses nested match expressions instead, generating O(n) AST nodes: - Before: 26 fields = ~102 seconds compile time - After: 26 fields = ~0.7 seconds compile time The fix is minimal and surgical - only records.ml is changed. It preserves all existing behavior and passes all tests. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com> * fix: prioritize @spice.default over is_option in pattern matching For a field like `field: option<int> [@spice.default Some(1)]`, the previous pattern matching order would match `| _, true, _` (is_option=true) before `| _, _, Some d` (has default), causing the default value to be ignored. Reordered patterns to check for default values first: 1. `| _, _, Some d` - If there's ANY default, use it 2. `| true, _, None` - Optional field, no default -> Ok None 3. `| _, true, None` - Option type, no default -> Ok None 4. `| _, _, None` - Required field, no default -> error Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com> * changelog --------- Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com> Co-authored-by: mununki <woonki.moon@gmail.com>
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‎CHANGELOG.md‎

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## 0.3.2(unreleased)
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- Fixes O(n) nested match decoder replaces O(n²) tuple pattern matching to prevent the hang with the recode with many fields https://github.com/green-labs/ppx_spice/pull/111
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## 0.3.1
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- Fixes [#107](https://github.com/green-labs/ppx_spice/issues/107) Arrays being reversed by Spice.arrayFromJson

‎src/ppx/records.ml‎

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@@ -43,73 +43,12 @@ let generate_encoder decls unboxed =
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|> Exp.fun_ Asttypes.Nolabel None [%pat? v]
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|> Utils.expr_func ~arity:1
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let generate_flat_decoder_expr decls =
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(* O(n) nested match decoder - replaces O(n²) tuple pattern matching *)
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let generate_nested_decoder decls =
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let loc = !default_loc in
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let dict_expr = Exp.ident (mknoloc (Longident.Lident "dict")) in
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let field_results =
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List.map
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(fun d ->
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let { name; key; codecs; default; is_optional; is_option } = d in
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let result_name = name ^ "_result" in
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let decode_expr =
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match codecs with
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| _, Some decode ->
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let get_expr = [%expr Dict.get [%e dict_expr] [%e key]] in
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let decode_applied = [%expr [%e decode]] in
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let opt_map =
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[%expr Option.map [%e get_expr] [%e decode_applied]]
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in
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let default_expr =
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match (is_optional, is_option, default) with
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| true, _, Some d ->
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[%expr Option.getOr [%e opt_map] (Ok [%e d])]
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| true, _, None -> [%expr Option.getOr [%e opt_map] (Ok None)]
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| _, true, _ -> [%expr Option.getOr [%e opt_map] (Ok None)]
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| _, _, Some d -> [%expr Option.getOr [%e opt_map] (Ok [%e d])]
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| _, _, None ->
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[%expr
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Option.getOr [%e opt_map]
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(Spice.error ([%e key] ^ " missing") v)]
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in
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default_expr
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| _ -> [%expr Spice.error ([%e key] ^ " missing") v]
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in
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(result_name, decode_expr))
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decls
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in
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let let_bindings =
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List.map
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(fun (result_name, decode_expr) ->
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Vb.mk (Pat.var (mknoloc result_name)) decode_expr)
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field_results
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in
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let tuple_expr =
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match field_results with
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| [ (result_name, _) ] -> Exp.ident (mknoloc (Longident.Lident result_name))
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| _ ->
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Exp.tuple
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(List.map
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(fun (result_name, _) ->
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Exp.ident (mknoloc (Longident.Lident result_name)))
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field_results)
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in
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let ok_pattern =
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match decls with
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| [ d ] ->
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let { name; _ } = d in
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Pat.construct
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(mknoloc (Longident.Lident "Ok"))
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(Some (Pat.var (mknoloc name)))
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| _ ->
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Pat.tuple
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(List.map
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(fun d ->
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let { name; _ } = d in
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Pat.construct
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(mknoloc (Longident.Lident "Ok"))
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(Some (Pat.var (mknoloc name))))
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decls)
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in
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(* Build the final Ok expression with the record *)
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let ok_expr =
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let record_fields =
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List.map
@@ -121,39 +60,60 @@ let generate_flat_decoder_expr decls =
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in
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[%expr Ok [%e Exp.record record_fields None]]
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in
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let error_patterns =
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List.mapi
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(fun i d ->
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let { key; _ } = d in
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let pats =
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List.init (List.length decls) (fun j ->
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if i = j then
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Pat.construct
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(mknoloc (Longident.Lident "Error"))
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(Some
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(Pat.constraint_
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(Pat.var (mknoloc "e"))
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(Typ.constr
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(mknoloc (Longident.parse "Spice.decodeError"))
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[])))
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else Pat.any ())
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in
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let pat = match pats with [ p ] -> p | _ -> Pat.tuple pats in
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Exp.case pat [%expr Spice.error ~path:[%e key] e.message e.value])
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decls
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(* Generate decode expression for a single field *)
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let generate_decode_expr d =
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let { key; codecs; default; is_optional; is_option; _ } = d in
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match codecs with
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| _, Some decode ->
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let get_expr = [%expr Dict.get [%e dict_expr] [%e key]] in
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let decode_applied = [%expr [%e decode]] in
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let opt_map = [%expr Option.map [%e get_expr] [%e decode_applied]] in
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(match (is_optional, is_option, default) with
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| _, _, Some d -> [%expr Option.getOr [%e opt_map] (Ok [%e d])]
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| true, _, None -> [%expr Option.getOr [%e opt_map] (Ok None)]
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| _, true, None -> [%expr Option.getOr [%e opt_map] (Ok None)]
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| _, _, None ->
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[%expr
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Option.getOr [%e opt_map] (Spice.error ([%e key] ^ " missing") v)])
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| _ -> [%expr Spice.error ([%e key] ^ " missing") v]
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in
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let match_expr =
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Exp.match_ tuple_expr (Exp.case ok_pattern ok_expr :: error_patterns)
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(* Build nested matches from the last field backwards *)
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let build_nested_matches decls inner_expr =
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let rec loop acc = function
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| [] -> acc
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| d :: rest ->
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let { name; key; _ } = d in
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let decode_expr = generate_decode_expr d in
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let var_pat = Pat.var (mknoloc name) in
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let ok_case =
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Exp.case
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(Pat.construct (mknoloc (Longident.Lident "Ok")) (Some var_pat))
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acc
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in
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let error_case =
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Exp.case
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(Pat.construct
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(mknoloc (Longident.Lident "Error"))
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(Some
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(Pat.constraint_
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(Pat.var (mknoloc "e"))
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(Typ.constr
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(mknoloc (Longident.parse "Spice.decodeError"))
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[]))))
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[%expr Spice.error ~path:[%e key] e.message e.value]
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in
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let match_expr = Exp.match_ decode_expr [ ok_case; error_case ] in
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loop match_expr rest
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in
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loop inner_expr (List.rev decls)
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in
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List.fold_right
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(fun vb acc -> Exp.let_ Nonrecursive [ vb ] acc)
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let_bindings match_expr
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let generate_nested_switches_recurse _path decls _remaining_decls =
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generate_flat_decoder_expr decls
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build_nested_matches decls ok_expr
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let generate_nested_switches decls =
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generate_nested_switches_recurse [] decls decls
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generate_nested_decoder decls
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let generate_decoder decls unboxed =
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match unboxed with

‎test/src/LargeRecordRepro.res‎

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// Reproduction of exponential compile time bug
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// Compile time grows ~4x per 2 additional fields:
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// - 20 fields: ~1s
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// - 22 fields: ~4s
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// - 24 fields: ~17s
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// - 26 fields: ~71s
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// - 28 fields: >180s (timeout)
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// - 31 fields: effectively infinite
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//
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// The issue is in the generated decoder code which creates nested switch
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// statements with exponential complexity in the ReScript type checker.
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@spice
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type t = {
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f1: option<string>,
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f2: option<string>,
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f3: option<string>,
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f4: option<string>,
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f5: option<string>,
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f6: option<string>,
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f7: option<string>,
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f8: option<string>,
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f9: option<string>,
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f10: option<string>,
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f11: option<string>,
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f12: option<string>,
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f13: option<string>,
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f14: option<string>,
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f15: option<string>,
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f16: option<string>,
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f17: option<string>,
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f18: option<string>,
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f19: option<string>,
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f20: option<string>,
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f21: option<string>,
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f22: option<string>,
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f23: option<string>,
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f24: option<string>,
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f25: option<string>,
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f26: option<string>,
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}

‎test/src/SmallRecord.res‎

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// 20 fields - should compile in ~1s
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@spice
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type t = {
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f1: option<string>,
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f2: option<string>,
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f3: option<string>,
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f4: option<string>,
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f5: option<string>,
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f6: option<string>,
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f7: option<string>,
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f8: option<string>,
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f9: option<string>,
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f10: option<string>,
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f11: option<string>,
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f12: option<string>,
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f13: option<string>,
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f14: option<string>,
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f15: option<string>,
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f16: option<string>,
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f17: option<string>,
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f18: option<string>,
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f19: option<string>,
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f20: option<string>,
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}

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