Use impossible and |?

lib/Extensions.fs

@@ -1,6 +1,12 @@
 [<AutoOpen>]
 module Ts2Ml.Extensions
 
+let inline (|?) (xo: 'a option) (y: 'a) : 'a = Option.defaultValue y xo
+
+/// Use when a certain code path is impossible or unreachable.
+let impossible fmt =
+  Printf.ksprintf (fun msg -> failwith ("impossible: " + msg)) fmt
+
 open System
 
 module Enum =

lib/JsHelper.fs

@@ -21,7 +21,7 @@ let getPackageJsonPath (exampleFilePath: string) =
           match rest with
           | userName :: packageName :: _ when userName.StartsWith("@") -> [userName; packageName]
           | packageName :: _ -> [packageName]
-          | _ -> failwith "impossible_getPackageJsonPath_root"
+          | _ -> impossible "getPackageJsonPath_root"
         let path =
           prefix @ packageName @ ["package.json"] |> String.concat Path.separator
 
@@ -163,7 +163,7 @@ let getJsModuleName (info: Syntax.PackageInfo option) (sourceFile: Path.Absolute
     match List.rev rest with
     | "index.d.ts" :: name :: _ -> name
     | name :: _ -> stripExtension name
-    | [] -> failwith "impossible"
+    | [] -> impossible "getJsModuleName_getSubmodule"
   match info with
   | Some info ->
     if info.indexFile |> Option.exists ((=) sourceFile) then

lib/Parser.fs

@@ -251,7 +251,7 @@ module private ParserImpl =
       |> Array.toList
 
   let readJSDocTag (tag: Ts.JSDocTag) : Comment =
-    let text = tag.comment |> Option.map readCommentText |> Option.defaultValue []
+    let text = tag.comment |> Option.map readCommentText |? []
     match tag.kind with
     | Kind.JSDocParameterTag ->
       let tag = tag :?> Ts.JSDocParameterTag
@@ -376,7 +376,7 @@ module private ParserImpl =
         match t.kind with
         | Kind.TypeReference -> !!(t :?> Ts.TypeReferenceNode).typeName
         | Kind.ExpressionWithTypeArguments -> !!(t :?> Ts.ExpressionWithTypeArguments).expression
-        | _ -> failwith "impossible"
+        | _ -> impossible "readTypeNode_TypeReference"
       match readIdentOrTypeVar ctx typrm lhs with
       | Choice1Of2 lt ->
         match t.typeArguments with
@@ -508,7 +508,7 @@ module private ParserImpl =
     { args = args; isVariadic = isVariadic; returnType = retType; loc = Node.location parent }
 
   and readMemberAttribute (ctx: ParserContext) (nd: Ts.NamedDeclaration) : MemberAttribute =
-    let accessibility = getAccessibility nd.modifiers |> Option.defaultValue Public
+    let accessibility = getAccessibility nd.modifiers |? Public
     let isStatic = hasModifier Kind.StaticKeyword nd.modifiers
     let comments = readCommentsForNamedDeclaration ctx nd
     { accessibility = accessibility; isStatic = isStatic; comments = comments; loc = Node.location nd }
@@ -675,7 +675,7 @@ module private ParserImpl =
     {
       comments = readCommentsForNamedDeclaration ctx i
       name = Name name
-      accessibility = getAccessibility i.modifiers |> Option.defaultValue Public
+      accessibility = getAccessibility i.modifiers |? Public
       typeParams = typrms
       implements = readInherits typrmsSet ctx i.heritageClauses
       isInterface = true
@@ -689,8 +689,8 @@ module private ParserImpl =
     let typrmsSet = typrms |> List.map (fun tp -> tp.name) |> Set.ofList
     {
       comments = readCommentsForNamedDeclaration ctx i
-      name = i.name |> Option.map (fun id -> Name id.text) |> Option.defaultValue ExportDefaultUnnamedClass
-      accessibility = getAccessibility i.modifiers |> Option.defaultValue Public
+      name = i.name |> Option.map (fun id -> Name id.text) |? ExportDefaultUnnamedClass
+      accessibility = getAccessibility i.modifiers |? Public
       typeParams = typrms
       implements = readInherits typrmsSet ctx i.heritageClauses
       isInterface = false
@@ -925,11 +925,11 @@ module private ParserImpl =
     let desc =
       doc.comment
       |> Option.map (readCommentText >> Description >> List.singleton)
-      |> Option.defaultValue []
+      |? []
     let tags =
       doc.tags
       |> Option.map (Array.map readJSDocTag >> List.ofArray)
-      |> Option.defaultValue []
+      |? []
     desc @ tags
 
   let readJSDoc (ctx: ParserContext) (doc: Ts.JSDoc) : Statement option =

lib/Typer.fs

@@ -222,7 +222,7 @@ module FullName =
     | Some info ->
       info.definitionsMap
       |> Trie.tryFind fullName.name
-      |> Option.defaultValue []
+      |? []
 
   let getExportType (ctx: TyperContext<_, _>) (fullName: FullName) : ExportType option =
     match ctx.info |> Map.tryFind fullName.source with
@@ -236,7 +236,7 @@ module FullName =
     | Definition.EnumCase _ -> Kind.OfEnumCase
     | Definition.Module m -> if m.isNamespace then Kind.OfNamespace else Kind.OfModule
     | Definition.Variable _ | Definition.Function _ -> Kind.OfValue
-    | Definition.Import (c, _) -> c.kind |> Option.map Set.toList |> Option.defaultValue []
+    | Definition.Import (c, _) -> c.kind |> Option.map Set.toList |? []
     | Definition.Member _ -> Kind.OfMember
 
   let hasKind (ctx: TyperContext<_, _>) (kind: Kind) (fullName: FullName) =
@@ -422,8 +422,8 @@ module Type =
   let rec findTypesInFieldLike pred s (fl: FieldLike) = findTypes pred s fl.value
   and findTypesInTypeParam pred s (tp: TypeParam) =
     seq {
-      yield! tp.extends |> Option.map (findTypes pred s) |> Option.defaultValue Seq.empty
-      yield! tp.defaultType |> Option.map (findTypes pred s) |> Option.defaultValue Seq.empty
+      yield! tp.extends |> Option.map (findTypes pred s) |? Seq.empty
+      yield! tp.defaultType |> Option.map (findTypes pred s) |? Seq.empty
     }
   and findTypesInFuncType pred s (ft: FuncType<Type>) =
     seq {
@@ -1080,8 +1080,8 @@ module Statement =
     and treatTypeParameters (state: {| origin: AnonymousInterfaceOrigin; namespace_: string list |}) (tps: TypeParam list) =
       seq {
         for tp in tps do
-          yield! tp.extends |> Option.map (findTypes typeFinder state) |> Option.defaultValue Seq.empty
-          yield! tp.defaultType |> Option.map (findTypes typeFinder state) |> Option.defaultValue Seq.empty
+          yield! tp.extends |> Option.map (findTypes typeFinder state) |? Seq.empty
+          yield! tp.defaultType |> Option.map (findTypes typeFinder state) |? Seq.empty
       }
     and treatNamed (state: {| origin: AnonymousInterfaceOrigin; namespace_: string list |}) name value =
       findTypes typeFinder {| state with origin = { state.origin with valueName = Some name } |} value
@@ -1442,7 +1442,7 @@ module ResolvedUnion =
       let inline getLiteralFieldsFromClass c = getLiteralFieldsFromClass getLiteralFieldsFromType c
       match ty with
       | Intrinsic | PolymorphicThis | TypeVar _ | Prim _ | TypeLiteral _ | Tuple _ | Func _ | NewableFunc _ -> Map.empty
-      | Erased _ -> failwith "impossible_getDiscriminatedFromUnion_getLiteralFieldsFromType_Erased"
+      | Erased _ -> impossible "getDiscriminatedFromUnion_getLiteralFieldsFromType_Erased"
       | Union u ->
         let result = u.types |> List.map getLiteralFieldsFromType
         result |> List.fold (fun state fields ->
@@ -1708,7 +1708,7 @@ let mergeSources newFileName (srcs: SourceFile list) =
     srcs |> List.map (fun src -> src.fileName, newFileName) |> Map.ofList
   let f (i: Ident) =
     i |> Ident.mapSource (fun path ->
-      sourceMapping |> Map.tryFind path |> Option.defaultValue path
+      sourceMapping |> Map.tryFind path |? path
     )
   let statements =
     srcs

src/Extensions.fs

@@ -59,7 +59,7 @@ type Argv<'T> with
         :> Argv<'U>
   member this.addFlag (key: string, f: 'U -> bool, ?descr, ?defaultValue, ?defaultDescr, ?alias) =
     this.boolean(!^key)
-        .addImpl<bool>(key, descr, dv=(defaultValue |> Option.defaultValue false), ?dd=defaultDescr, ?alias=alias)
+        .addImpl<bool>(key, descr, dv=(defaultValue |? false), ?dd=defaultDescr, ?alias=alias)
         :> Argv<'U>
   member this.addCounter (key: string, f: 'U -> int, ?descr, ?alias) =
     this.count(!^key)