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Explanation: You can ❰merge❱ the alternatives of a union using a record with one
handler per alternative, like this:
┌─────────────────────────────────────────────────────────────────────┐
│ let union = < Left = +2 | Right : Bool > │
│ in let handlers = { Left = Natural/even, Right = λ(x : Bool) → x } │
│ in merge handlers union : Bool │
└─────────────────────────────────────────────────────────────────────┘
... as long as the output type of each handler function matches the declared type
of the result:
┌───────────────────────────────────────────────────────────┐
│ handlers : { Left : Natural → Bool, Right : Bool → Bool } │
└───────────────────────────────────────────────────────────┘
⇧ ⇧
These output types ...
... must match the declared type of the ❰merge❱
⇩
┌─────────────────────────────┐
│ merge handlers union : Bool │
└─────────────────────────────┘
For example, the following expression is $_NOT valid:
┌──────────────────────────────────────────────────────────────────────┐
│ let union = < Left = +2 | Right : Bool > │
│ in let handlers = { Left = Natural/even, Right = λ(x : Bool) → x } │
│ in merge handlers union : Text │
└──────────────────────────────────────────────────────────────────────┘
⇧
Invalid: Doesn't match output of either handler
Your handler for the following alternative:
↳ $txt0
... needs to return an output value of type:
↳ $txt1
... but actually returns an output value of a different type:
↳ $txt2
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