| | 140 | |
| | 141 | |
| | 142 | === Changing the record constructor === |
| | 143 | |
| | 144 | The `set` method continues with the as-is record constructor, with no attempt to figure out the 'appropriate' constructor for the fields names presented. This follows H98 behaviour. |
| | 145 | |
| | 146 | So if in update syntax you present field names which are not in the current constructor (but are in other constructors for the same type), you'll get pattern match failure (`Non-exhaustive patterns in record update`). For example: |
| | 147 | {{{ |
| | 148 | data Txyz = Txyz { x :: Int, y :: String, z :: Bool } |
| | 149 | | Tyx { y :: String, x :: Int } |
| | 150 | deriving (Show, Read, Eq) |
| | 151 | |
| | 152 | tyx = Tyx{ y = "hello", x = 72 } |
| | 153 | |
| | 154 | txyz = tyx{ z = False } -- run-time fail: Non-exhaustive patterns in record update |
| | 155 | -- no attempt to change to constructor Txyz |
| | 156 | }}} |
| | 157 | |
| | 158 | To achieve this, you'd need to put an explicit data constructor (presumably using punning/wildcards): |
| | 159 | {{{ |
| | 160 | case tyx of { Tyx{..} -> Txyz{ z = False, .. } } |
| | 161 | }}} |
