Use typescript type predicate functions

Author: lpil

compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bools__shadowed_bools_and_nil_typescript.snap

@@ -1,6 +1,8 @@
 ---
 source: compiler-core/src/javascript/tests/bools.rs
+assertion_line: 81
 expression: "\npub type True { True False Nil }\npub fn go(x, y) {\n  let assert True = x\n  let assert False = x\n  let assert Nil = y\n}\n"
+snapshot_kind: text
 ---
 ----- SOURCE CODE
 
@@ -17,15 +19,15 @@ import type * as _ from "../gleam.d.mts";
 
 export class True extends _.CustomType {}
 export function True$True(): True$;
-export function True$isTrue(value: True$): boolean;
+export function True$isTrue(value: any): value is True$;
 
 export class False extends _.CustomType {}
 export function True$False(): True$;
-export function True$isFalse(value: True$): boolean;
+export function True$isFalse(value: any): value is True$;
 
 export class Nil extends _.CustomType {}
 export function True$Nil(): True$;
-export function True$isNil(value: True$): boolean;
+export function True$isNil(value: any): value is True$;
 
 export type True$ = True | False | Nil;
 

compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__const_with_fields_typescript.snap

@@ -26,7 +26,7 @@ export class Mine extends _.CustomType {
   b: number;
 }
 export function Mine$Mine(a: number, b: number): Mine$;
-export function Mine$isMine(value: Mine$): boolean;
+export function Mine$isMine(value: any): value is Mine$;
 export function Mine$Mine$0(value: Mine$): number;
 export function Mine$Mine$a(value: Mine$): number;
 export function Mine$Mine$1(value: Mine$): number;

compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__long_name_variant_mixed_labels_typescript.snap

@@ -44,8 +44,8 @@ export function TypeWithALongNameAndSeveralArguments$TypeWithALongNameAndSeveral
   b: string,
 ): TypeWithALongNameAndSeveralArguments$;
 export function TypeWithALongNameAndSeveralArguments$isTypeWithALongNameAndSeveralArguments(
-  value: TypeWithALongNameAndSeveralArguments$,
-): boolean;
+  value: any,
+): value is TypeWithALongNameAndSeveralArguments$;
 export function TypeWithALongNameAndSeveralArguments$TypeWithALongNameAndSeveralArguments$0(value: TypeWithALongNameAndSeveralArguments$): string;
 export function TypeWithALongNameAndSeveralArguments$TypeWithALongNameAndSeveralArguments$1(
   value: TypeWithALongNameAndSeveralArguments$,

compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unapplied_record_constructors_typescript.snap

@@ -22,7 +22,7 @@ export class Cat extends _.CustomType {
   name: string;
 }
 export function Cat$Cat(name: string): Cat$;
-export function Cat$isCat(value: Cat$): boolean;
+export function Cat$isCat(value: any): value is Cat$;
 export function Cat$Cat$0(value: Cat$): string;
 export function Cat$Cat$name(value: Cat$): string;
 

compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__custom_types__unnamed_fields_typescript.snap

@@ -24,7 +24,7 @@ export class Ip extends _.CustomType {
   0: string;
 }
 export function Ip$Ip($0: string): Ip$;
-export function Ip$isIp(value: Ip$): boolean;
+export function Ip$isIp(value: any): value is Ip$;
 export function Ip$Ip$0(value: Ip$): string;
 
 export type Ip$ = Ip;

compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__generics__record_generics_typescript.snap

@@ -25,7 +25,7 @@ export class Cat<I> extends _.CustomType {
   type_: I;
 }
 export function Animal$Cat<I>(type_: I): Animal$<I>;
-export function Animal$isCat<I>(value: Animal$<I>): boolean;
+export function Animal$isCat<I>(value: any): value is Animal$<unknown>;
 export function Animal$Cat$0<I>(value: Animal$<I>): I;
 export function Animal$Cat$type_<I>(value: Animal$<I>): I;
 
@@ -36,7 +36,7 @@ export class Dog<I> extends _.CustomType {
   type_: I;
 }
 export function Animal$Dog<I>(type_: I): Animal$<I>;
-export function Animal$isDog<I>(value: Animal$<I>): boolean;
+export function Animal$isDog<I>(value: any): value is Animal$<unknown>;
 export function Animal$Dog$0<I>(value: Animal$<I>): I;
 export function Animal$Dog$type_<I>(value: Animal$<I>): I;
 

compiler-core/src/javascript/typescript.rs

@@ -510,12 +510,8 @@ impl<'a> TypeScriptGenerator<'a> {
             type_name_with_generics,
             type_parameters,
         );
-        let variant_check_definition = self.variant_check_definition(
-            constructor,
-            type_name,
-            type_name_with_generics,
-            type_parameters,
-        );
+        let variant_check_definition =
+            self.variant_check_definition(constructor, type_name, type_parameters);
         let fields_definition = self.variant_fields_definition(
             constructor,
             type_name,
@@ -657,24 +653,35 @@ impl<'a> TypeScriptGenerator<'a> {
         &self,
         constructor: &'a TypedRecordConstructor,
         type_name: &'a str,
-        type_name_with_generics: &Document<'a>,
         type_parameters: &'a [Arc<Type>],
     ) -> Document<'a> {
         let function_name = eco_format!(
             "{type_name}$is{variant_name}",
             variant_name = constructor.name
         )
         .to_doc();
-
-        docvec![
+        let mut document = docvec![
             "export function ",
             name_with_generics(function_name, type_parameters),
             "(",
-            docvec![break_("", "",), "value: ", type_name_with_generics.clone(),].nest(INDENT),
+            docvec![break_("", "",), "value: any"].nest(INDENT),
             break_(",", ""),
-            "): boolean;",
-        ]
-        .group()
+            "): value is ",
+            type_name,
+            "$",
+        ];
+        if !type_parameters.is_empty() {
+            for i in 0..type_parameters.len() {
+                if i == 0 {
+                    document = document.append("<unknown");
+                } else {
+                    document = document.append(", unknown");
+                }
+            }
+            document = document.append('>');
+        };
+        document = document.append(';');
+        document.group()
     }
 
     fn variant_fields_definition(

compiler-core/templates/prelude.d.mts

@@ -31,8 +31,8 @@ export const List: {
 }
 export function List$Empty<T>(): List<T>;
 export function List$NonEmpty<T>(head: T, tail: List<T>): List<T>;
-export function List$isEmpty(list: any): boolean;
-export function List$isNonEmpty(list: any): boolean;
+export function List$isEmpty(list: any): list is List<unknown>;
+export function List$isNonEmpty(list: any): list is List<unknown>;
 export function List$NonEmpty$first<T>(list: List<T>): T | undefined;
 export function List$NonEmpty$rest<T>(list: List<T>): List<T> | undefined;
 /** @deprecated */
@@ -72,7 +72,7 @@ export function BitArray$BitArray(
   bitSize: number,
   bitOffset: number,
 ): BitArray;
-export function BitArray$isBitArray(value: any): boolean;
+export function BitArray$isBitArray(value: any): value is BitArray;
 export function BitArray$BitArray$data(value: BitArray): DataView;
 
 export interface UtfCodepoint {
@@ -97,8 +97,8 @@ export const Result: {
 }
 export function Result$Ok<T, E>(value: T): Result<T, E>;
 export function Result$Error<T, E>(error: E): Result<T, E>;
-export function Result$isError(data: any): boolean;
-export function Result$isOk(data: any): boolean;
+export function Result$isError(data: any): data is Result<unknown, unknown>;
+export function Result$isOk(data: any): data is Result<unknown, unknown>;
 export function Result$Ok$0<T, E>(result: Result<T, E>): T | undefined;
 export function Result$Error$0<T, E>(result: Result<T, E>): E | undefined;
 /** @deprecated */