I'm going to add a trait that provides .join() -> Box<dyn Template>.
wrap_template() should handle it transparently, but the current interface
would require excessive boxing.
This involves a little hack to insert a lambda parameter 'x' to be used at
keyword position. If the template language were dynamically typed (and were
interpreted), .map() implementation would be simpler. I considered that, but
interpreter version has its own warts (late error reporting, uneasy to cache
static object, etc.), and I don't think the current template engine is
complex enough to rewrite from scratch.
.map() returns template, which can't be join()-ed. This will be fixed later.
A lambda expression will be allowed only in .map() operation. The syntax is
borrowed from Rust closure.
In Mercurial, a map operation is implemented by context substitution. For
example, 'parents % "{node}"' prints parents[i].node for each. There are two
major problems: 1. the top-level context cannot be referred from the inner map
expression. 2. context of different types inserts arbitrarily-named keywords
(e.g. a dict type inserts "{key}" and "{value}", but how we could know.)
These issues should be avoided by using explicitly named parameters.
parents.map(|parent| parent.commit_id ++ " " ++ commit_id)
^^^^^^^^^ global keyword
A downside is that we can't reuse template fragment in map expression. Suppose
we have -T commit_summary, -T 'parents.map(commit_summary)' doesn't work.
# only usable as a top-level template
'commit_summary' = 'commit_id.short() ++ " " ++ description.first_line()'
Another problem is that a lambda expression might be confused with an alias
function.
# .map(f) doesn't work, but .map(g) does
'f(x)' = 'x'
'g' = '|x| x'
I'm going to add a lambda expression, and the current type-error message
wouldn't work for the lambda type. I also renamed "argument" to "expression"
as the expect_<type>() helper may be called against any expression node.
I'm thinking of rewriting the evaluation part as a simple interpreter. It
will increase the runtime cost (about a few microseconds per entry I suppose),
but will greatly reduce the complexity of generic property function chaining.
The extracted template_builder module is the part I'm going to reimplement.
