Added textToId function

Fixes #432

Guide/recipes.markdown

@@ -169,6 +169,21 @@ In case the id is hardcoded, you can just type UUID value with the right type si
 let projectId = "ca63aace-af4b-4e6c-bcfa-76ca061dbdc6" :: Id Project
 ```
 
+## Getting a `Id Something` from a `Text` / `ByteString` / `String`
+
+Sometimes you have a text, bytestring or string which represents some record id. You can transform it to an Id like this:
+
+```haskell
+let myUUID :: Text = ...
+let projectId = textToId myUUID
+```
+
+In case the id is hardcoded, you can just type UUID value with the right type signature like this:
+
+```haskell
+let projectId = "ca63aace-af4b-4e6c-bcfa-76ca061dbdc6" :: Id Project
+```
+
 ## Making a dynamic Login/Logout button
 
 Depending on the `user` object from the viewContext, we can tell that there is no user logged in when the `user` is `Nothing`, and confirm someone is logged in if the `user` is a `Just user`. Here is an example of a navbar, which has a dynamic Login/Logout button. You can define this in your View/Layout to reuse this in your Views.

IHP/ModelSupport.hs

@@ -14,7 +14,7 @@ import Database.PostgreSQL.Simple.FromField hiding (Field, name)
 import Database.PostgreSQL.Simple.ToField
 import Data.Default
 import Data.Time.Format.ISO8601 (iso8601Show)
-import Data.String.Conversions (cs)
+import Data.String.Conversions (cs ,ConvertibleStrings)
 import Data.Time.Clock
 import Data.Time.Calendar
 import Unsafe.Coerce
@@ -205,9 +205,27 @@ instance Newtype.Newtype (Id' model) where
     pack = Id
     unpack (Id uuid) = uuid
 
+-- | Sometimes you have a hardcoded UUID value which represents some record id. This instance allows you
+-- to write the Id like a string:
+--
+-- > let projectId = "ca63aace-af4b-4e6c-bcfa-76ca061dbdc6" :: Id Project
 instance Read (PrimaryKey model) => IsString (Id' model) where
     fromString uuid = Id (Prelude.read uuid)
 
+-- | Transforms a text, bytestring or string into an Id. Throws an exception if the input is invalid.
+--
+-- __Example:__
+--
+-- > let projectIdText = "7cbc76e2-1c4f-49b6-a7d9-5015e7575a9b" :: Text
+-- > let projectId = (textToId projectIdText) :: Id Project
+--
+-- In case your UUID value is hardcoded, there is also an 'IsString' instance, so you
+-- can just write it like:
+--
+-- > let projectId = "ca63aace-af4b-4e6c-bcfa-76ca061dbdc6" :: Id Project
+textToId :: (Read (PrimaryKey model), ConvertibleStrings text String) => text -> Id' model
+textToId text = Id (Prelude.read (cs text))
+
 instance Default (PrimaryKey model) => Default (Id' model) where
     {-# INLINE def #-}
     def = Newtype.pack def