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templater: update comment why Template isn't TemplateProperty<Output = ..>

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They are similar in a way that both of them can represent dynamic/static
evaluation, but their behaviors are different in error handling.
This commit is contained in:
Yuya Nishihara 2024-03-23 11:40:49 +09:00
parent 577e030db2
commit df7be43ab6
1 changed files with 10 additions and 5 deletions
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15
cli/src/template_builder.rs
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@ -134,11 +134,16 @@ pub enum CoreTemplatePropertyKind<'a> {
Timestamp(Box<dyn TemplateProperty<Output = Timestamp> + 'a>),
TimestampRange(Box<dyn TemplateProperty<Output = TimestampRange> + 'a>),
// TODO: This argument no longer makes sense. Maybe we can migrate these to
// TemplateProperty<..>:
// Similar to `TemplateProperty<I, Output = Box<dyn Template<()> + 'a>`, but doesn't
// capture `I` to produce `Template<()>`. The context `I` would have to be cloned
// to convert `Template<I>` to `Template<()>`.
// Both TemplateProperty and Template can represent a value to be evaluated
// dynamically, which suggests that `Box<dyn Template + 'a>` could be
// composed as `Box<dyn TemplateProperty<Output = Box<dyn Template ..`.
// However, there's a subtle difference: TemplateProperty is strict on
// error, whereas Template is usually lax and prints an error inline. If
// `concat(x, y)` were a property returning Template, and if `y` failed to
// evaluate, the whole expression would fail. In this example, a partial
// evaluation output is more useful. That's one reason why Template isn't
// wrapped in a TemplateProperty. Another reason is that the outermost
// caller expects a Template, not a TemplateProperty of Template output.
Template(Box<dyn Template + 'a>),
ListTemplate(Box<dyn ListTemplate + 'a>),
}