allow message type to be exception

src/FsSpreadsheet/DSL/CellBuilder.fs

@@ -32,7 +32,7 @@ type CellBuilder() =
 
     member this.SignMessages (messages : Message list) : Message list =
         messages
-        |> List.map (sprintf "In Cell: %s")
+        |> List.map (fun m -> m.MapText (sprintf "In Cell: %s"))
 
     member inline this.Yield(n: RequiredSource<unit>) = 
         n
@@ -66,12 +66,12 @@ type CellBuilder() =
     member inline this.Yield(s : string option) : SheetEntity<Value list> =
         match s with
         | Option.Some s -> this.Yield s
-        | None -> NoneRequired ["Value is missing"]
+        | None -> NoneRequired [message "Value is missing"]
 
     member inline this.Yield(n: 'a option when 'a :> System.IFormattable) = 
         match n with
         | Option.Some s -> this.Yield s
-        | None -> NoneRequired ["Value is missing"]
+        | None -> NoneRequired [message "Value is missing"]
 
     member inline this.YieldFrom(ns: SheetEntity<Value list> seq) =   
         ns
@@ -154,15 +154,15 @@ type CellBuilder() =
             | NoneRequired m -> NoneOptional m
             | se -> se
         with
-        | err -> NoneOptional [err.Message]
+        | err -> NoneOptional [message err.Message]
 
     member inline this.Run(children: Expr<RequiredSource<SheetEntity<Value list>>>) =
         try 
             match this.AsCellElement ((eval<RequiredSource<SheetEntity<Value list>>> children).Source) with
             | NoneOptional m -> NoneRequired m
             | se -> se
         with
-        | err -> NoneOptional [err.Message]
+        | err -> NoneOptional [message err.Message]
 
     member inline this.Run(children: Expr<SheetEntity<Value list>>) =
         this.AsCellElement (eval<SheetEntity<Value list>> children)

src/FsSpreadsheet/DSL/ColumnBuilder.fs

@@ -22,7 +22,7 @@ type ColumnBuilder() =
 
     member this.SignMessages (messages : Message list) : Message list =
         messages
-        |> List.map (sprintf "In Column: %s")
+        |> List.map (fun m -> m.MapText (sprintf "In Column: %s"))
 
     member inline _.Yield(c: ColumnElement) =
         SheetEntity.ok [c]
@@ -175,15 +175,15 @@ type ColumnBuilder() =
             | NoneRequired m -> NoneOptional m
             | se -> se
         with
-        | err -> NoneOptional [err.Message]
+        | err -> NoneOptional [message err.Message]
 
     member inline this.Run(children: Expr<RequiredSource<SheetEntity<ColumnElement list>>>) =
         try 
             match (eval<RequiredSource<SheetEntity<ColumnElement list>>> children).Source with
             | NoneOptional m -> NoneRequired m
             | se -> se
         with
-        | err -> NoneOptional [err.Message]
+        | err -> NoneOptional [message err.Message]
 
     member inline this.Run(children: Expr<SheetEntity<ColumnElement list>>) =
         (eval<SheetEntity<ColumnElement list>> children).Value

src/FsSpreadsheet/DSL/DSL.fs

@@ -39,7 +39,7 @@ type DSL =
             DSL.opt elem
         with
         | err -> 
-            NoneOptional([err.Message])
+            NoneOptional([message err.Message])
 
     /// Drops the cell with the given message
     static member dropCell message : SheetEntity<Value> = NoneRequired [message]

src/FsSpreadsheet/DSL/Operators.fs

@@ -57,14 +57,14 @@ module Operators =
     ///
     /// If expression does fail, returns a missing required value
     let inline (!!) (v : 'T) : SheetEntity<Value> =
-        let f = fun s -> NoneRequired([s])
+        let f = fun s -> NoneRequired([message s])
         parseAny f v
 
     /// Optional value operator
     ///
     /// If expression does fail, returns a missing optional value
     let inline (!?) (v : 'T) : SheetEntity<Value> =
-        let f = fun s -> NoneOptional([s])
+        let f = fun s -> NoneOptional([message s])
         parseAny f v 
 
     /// Optional operators for cell, row, column and sheet expressions

src/FsSpreadsheet/DSL/RowBuilder.fs

@@ -19,7 +19,7 @@ type RowBuilder() =
 
     member this.SignMessages (messages : Message list) : Message list =
         messages
-        |> List.map (sprintf "In Row: %s")
+        |> List.map (fun m -> m.MapText (sprintf "In Row: %s"))
 
     member inline this.Yield(n: RequiredSource<unit>) = 
         n
@@ -163,15 +163,15 @@ type RowBuilder() =
             | NoneRequired m -> NoneOptional m
             | se -> se
         with
-        | err -> NoneOptional [err.Message]
+        | err -> NoneOptional [message err.Message]
 
     member inline this.Run(children: Expr<RequiredSource<SheetEntity<RowElement list>>>) =
         try 
             match (eval<RequiredSource<SheetEntity<RowElement list>>> children).Source with
             | NoneOptional m -> NoneRequired m
             | se -> se
         with
-        | err -> NoneOptional [err.Message]
+        | err -> NoneOptional [message err.Message]
 
     member inline this.Run(children: Expr<SheetEntity<RowElement list>>) =
         (eval<SheetEntity<RowElement list>> children).Value

src/FsSpreadsheet/DSL/SheetBuilder.fs

@@ -16,7 +16,7 @@ type SheetBuilder(name : string) =
 
     member this.SignMessages (messages : Message list) : Message list =
         messages
-        |> List.map (sprintf "In Sheet %s: %s" name)
+        |> List.map (fun m -> m.MapText (sprintf "In Sheet %s: %s" name))
 
     member inline _.Yield(se: SheetElement) =
         SheetEntity.ok [se]

src/FsSpreadsheet/DSL/TableBuilder.fs

@@ -18,7 +18,7 @@ type TableBuilder(name : string) =
 
     member this.SignMessages (messages : Message list) : Message list =
         messages
-        |> List.map (sprintf "In Sheet %s: %s" name)
+        |> List.map (fun m -> m.MapText (sprintf "In Sheet %s: %s" name))
 
     member inline _.Yield(se: TableElement) =
         SheetEntity.ok [se]

src/FsSpreadsheet/DSL/Types.fs

@@ -2,7 +2,61 @@
 
 open FsSpreadsheet
 
-type Message = string
+[<AutoOpen>]
+type Message = 
+    | Text of string
+    | Exception of exn
+
+    static member message (s : string) = Text s
+
+    static member message (e : #exn) = Exception e
+
+    member this.MapText(m : string -> string) =
+        match this with
+        | Text s -> Text (m s)
+        | Exception e -> this
+
+    member this.AsString() = 
+        match this with
+        | Text s -> s
+        | Exception e -> e.Message
+
+    member this.TryText() = 
+        match this with
+        | Text s -> Some s
+        | _ -> None
+
+    member this.TryException() = 
+        match this with
+        | Exception e -> Some e
+        | _ -> None
+
+    member this.IsTxt = 
+        match this with
+        | Text s -> true
+        | _ -> false
+
+    member this.IsExc = 
+        match this with
+        | Text s -> true
+        | _ -> false
+
+
+module Messages =
+
+    let format (ms : Message list) =
+        ms
+        |> List.map (fun m -> m.AsString())
+        |> List.reduce (fun a b -> a + ";" + b)
+
+    let fail (ms : Message list) =
+        let s = format ms
+        if ms |> List.exists (fun m -> m.IsExc) then
+            printfn "s"
+            raise (ms |> List.pick (fun m -> m.TryException()))
+        else
+            failwith s
+
 
 [<AutoOpen>]
 type SheetEntity<'T> =
@@ -30,7 +84,7 @@ module SheetEntityExtensions =
             | NoneOptional ms | NoneRequired ms when ms = [] -> 
                 failwith $"SheetEntity of type {typeof<'T>.Name} does not contain Value."
             | NoneOptional ms | NoneRequired ms -> 
-                let appendedMessages = ms |> List.reduce (fun a b -> a + "\n\t" + b)
+                let appendedMessages = Messages.format ms
                 failwith $"SheetEntity of type {typeof<'T>.Name} does not contain Value: \n\t{appendedMessages}"
 
 type Value = DataType * string

src/FsSpreadsheet/DSL/WorkbookBuilder.fs

@@ -16,7 +16,7 @@ type WorkbookBuilder() =
 
     member this.SignMessages (messages : Message list) : Message list =
         messages
-        |> List.map (sprintf "In Workbook: %s")
+        |> List.map (fun m -> m.MapText (sprintf "In Workbook: %s"))
 
     member inline _.Yield(c: WorkbookElement) =
         SheetEntity.ok [c]