Lets's say I have two type aliases:
type alias A = {...}
type alias B = {...}
and a union type
type Content = A | B
and a model type
type alias Model= {cont : Content, ...}
init : Content -> Model
init cont = {cont = cont, ...}
How do I pass a record of type A to init.
a : A
a = {...}
init a
throws the following error:
Detected errors in 1 module.
## ERRORS in Main.elm ##########################################################
-- TYPE MISMATCH ------------------------------------------------------ Main.elm
The 1st argument to function `init` has an unexpected type.
78| init e
^
As I infer the type of values flowing through your program, I see a conflict
between these two types:
Content
A
I would imagine that A is a kind of "subtype" of Content. I can't just write
a:Content
a = {...}
init a
because some of the logic does case analysis on Content
Elm's union types are so-called discriminated unions (or tagged unions). The discriminating part is a tag or constructor. In this case I think you want:
type Content = TagA A | TagB B
Note that the first capitalised name is the tag, and anything else is other types or type variables.
Now you can do:
a : A
a = { ... }
init (TagA a)
Within init you can distinguish the two types by doing:
init content =
case content of
TagA a -> ...
TagB b -> ...
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