Example of unfriendly closure error messages

jadski-unfriendly-messages/Tricky.elm

@@ -0,0 +1,126 @@
+{-
+
+  Unhelpful error messages in Elm closures (deliberately not a minimal code example, to illustrate the message).
+
+-}
+module Tricky where
+
+import List
+import Dict
+import Math.Vector2 as V2
+
+type alias ParsedSvgData = { id       : String 
+                           , title    : String
+                           , polygons : List ( List ( Float, Float ) ) }
+
+type alias Country = { code  : String
+                     , name  : String
+                     , lines : List Line }
+
+type alias Line = ( { a1 : V2.Vec2 }, { a2 : V2.Vec2 } )
+
+scale = 1.2
+
+{- Eg. 1 : Record error - message identifies entire let (relatively easy to solve as record syntax reported) -}
+{-
+hacky1 : Dict.Dict String ParsedSvgData -> List Country
+hacky1 countryShapes =
+
+    let countryShapeList = Dict.toList countryShapes
+
+        makeVector ( x, y ) = V2.vec2 ( x / scale ) ( y / scale )
+
+        makeMoreComplicated something =
+            case something of
+                head :: tail -> ( { a1 = head }, { a2 = tail } )
+
+        makeVectors floatTupleList = List.map makeVector floatTupleList
+
+        makeTuples vectors = 
+            let helper v1 v2 = ( { a1 = v1 }, { a1 = v2 } )           {- invalid field here -}
+            in
+              case vectors of
+                _ :: tail -> List.map2 helper vectors ( tail )
+
+        makeLines polygons =
+            case polygons of
+              first :: rest -> makeTuples ( makeVectors first ) ++ ( makeLines rest )
+              _             -> []
+
+        makeCountry ( id, record ) =
+            { code  = id
+            , name  = record.title
+            , lines = makeLines record.polygons }
+
+    in
+      List.map makeCountry countryShapeList
+-}
+
+{- Eg. 2 : Type error - now you have a real headache, and may need to decompose your function   -}
+hacky2 : Dict.Dict String ParsedSvgData -> List Country
+hacky2 countryShapes =
+
+    let countryShapeList = Dict.toList countryShapes
+
+        makeVector ( x, y ) = ( ( x / scale ), ( y / scale ) ) {- Incorrect type here (should be Vec2) -}
+
+        makeMoreComplicated something =
+            case something of
+                head :: tail -> ( { a1 = head }, { a2 = tail } )
+
+        makeVectors floatTupleList = List.map makeVector floatTupleList
+
+        makeTuples vectors = 
+            let helper v1 v2 = ( { a1 = v1 }, { a2 = v2 } )
+            in
+              case vectors of
+                _ :: tail -> List.map2 helper vectors ( tail )
+
+        makeLines polygons =
+            case polygons of
+              first :: rest -> makeTuples ( makeVectors first ) ++ ( makeLines rest )
+              _             -> []
+
+        makeCountry ( id, record ) =
+            { code  = id
+            , name  = record.title
+            , lines = makeLines record.polygons }
+
+    in
+      List.map makeCountry countryShapeList
+
+{- Error free code - type declarations improve reporting immensely, but aren't always used when prototyping -}
+{-
+meticulous : Dict.Dict String ParsedSvgData -> List Country
+meticulous countryShapes =
+
+    let countryShapeList = Dict.toList countryShapes
+
+        makeVector : ( Float, Float ) -> V2.Vec2
+        makeVector ( x, y ) = V2.vec2 ( x / scale ) ( y / scale )
+
+        makeVectors : List ( Float, Float ) -> List V2.Vec2
+        makeVectors floatTupleList = List.map makeVector floatTupleList
+
+        makeTuples : List V2.Vec2 -> List Line
+        makeTuples vectors = 
+            let helper v1 v2 = ( { a1 = v1 }, {a2 = v2 } )
+            in
+              case vectors of
+                _ :: tail -> List.map2 helper vectors ( tail )
+
+        makeLines : List ( List ( Float, Float ) ) -> List Line
+        makeLines polygons =
+            case polygons of
+              first :: rest -> makeTuples ( makeVectors first ) ++ ( makeLines rest )
+              _             -> []
+
+        makeCountry : ( String, ParsedSvgData ) -> Country
+        makeCountry ( id, record ) =
+            { code  = id
+            , name  = record.title
+            , lines = makeLines record.polygons }
+
+    in
+      List.map makeCountry countryShapeList
+-}