path: root/doc/forum/How_to_create_a_property_with_info
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authorJoey Hess2017-08-25 19:22:26 -0400
committerJoey Hess2017-08-25 19:22:26 -0400
commit34501bb0ee449a6adff1082ab009febc48b8df85 (patch)
treec03db6eaf9a07e5626400af3d73814f8ac5de4a1 /doc/forum/How_to_create_a_property_with_info
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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2017-08-25T23:07:12Z"
+ content="""
+It's not allowed for the content of Info to come from an IO action.
+Info has to be static. This allows one Host to introspect the Info of
+another Host. The Dns properties rely on that.
+So, the type checker is right in preventing this. It's also not allowed
+to use ensureProperty with a property that HasInfo, as the info would
+not propigate to the outer property. The type checker is also preventing
+you making that mistake.
+(You also forgot to pass the `w` parameter to `ensureProperty`,
+which made the type checker unhappy as well and probably confused the error
+To accomplish your goal, you could use:
+ data DebOMaticHostMirror = DebOMaticHostMirror
+If a Host has this in its Info, you know that Host is the one with
+debomatic installed. You can then get its hostname using the `hostName`
+field accessor on the Host.
+The property that does that will need to be passed a `[Host]` which will
+typically be the `hosts` list defined in config.hs.