forgejo/templates
wxiaoguang 5b89670a31
Use a general Eval function for expressions in templates. (#23927)
One of the proposals in #23328

This PR introduces a simple expression calculator
(templates/eval/eval.go), it can do basic expression calculations.

Many untested template helper functions like `Mul` `Add` can be replaced
by this new approach.

Then these `Add` / `Mul` / `percentage` / `Subtract` / `DiffStatsWidth`
could all use this `Eval`.

And it provides enhancements for Golang templates, and improves
readability.

Some examples:

----

* Before: `{{Add (Mul $glyph.Row 12) 12}}`
* After: `{{Eval $glyph.Row "*" 12 "+" 12}}`

----

* Before: `{{if lt (Add $i 1) (len $.Topics)}}`
* After: `{{if Eval $i "+" 1 "<" (len $.Topics)}}`

## FAQ

### Why not use an existing expression package?

We need a highly customized expression engine:

* do the calculation on the fly, without pre-compiling
* deal with int/int64/float64 types, to make the result could be used in
Golang template.
* make the syntax could be used in the Golang template directly
* do not introduce too much complex or strange syntax, we just need a
simple calculator.
* it needs to strictly follow Golang template's behavior, for example,
Golang template treats all non-zero values as truth, but many 3rd
packages don't do so.

### What's the benefit?

* Developers don't need to add more `Add`/`Mul`/`Sub`-like functions,
they were getting more and more.
Now, only one `Eval` is enough for all cases.
* The new code reads better than old `{{Add (Mul $glyph.Row 12) 12}}`,
the old one isn't familiar to most procedural programming developers
(eg, the Golang expression syntax).
* The `Eval` is fully covered by tests, many old `Add`/`Mul`-like
functions were never tested.

### The performance?

It doesn't use `reflect`, it doesn't need to parse or compile when used
in Golang template, the performance is as fast as native Go template.

### Is it too complex? Could it be unstable?

The expression calculator program is a common homework for computer
science students, and it's widely used as a teaching and practicing
purpose for developers. The algorithm is pretty well-known.

The behavior can be clearly defined, it is stable.
2023-04-07 21:25:49 +08:00
..
admin Clean template/helper.go (#23922) 2023-04-07 03:31:41 -04:00
api/packages/pypi Remove incorrect HTML self close tag (#23748) 2023-03-27 18:05:51 +02:00
base Introduce GitHub markdown editor, keep EasyMDE as fallback (#23876) 2023-04-03 18:06:57 +08:00
code Use data-tooltip-content for tippy tooltip (#23649) 2023-03-24 18:35:38 +08:00
custom Add footer extra links template (#9576) 2020-01-03 20:41:56 +02:00
devtest Introduce GitHub markdown editor, keep EasyMDE as fallback (#23876) 2023-04-03 18:06:57 +08:00
explore Add ONLY_SHOW_RELEVANT_REPOS back, fix explore page bug, make code more strict (#23766) 2023-03-29 08:41:45 -05:00
mail Remove incorrect HTML self close tag (#23748) 2023-03-27 18:05:51 +02:00
org Org pages style fixes (#23901) 2023-04-04 06:49:09 +02:00
package More specific and unique feed name for NuGet install command template. (#23889) 2023-04-06 17:35:32 -04:00
projects Introduce GiteaLocaleNumber custom element to handle number localization on pages. (#23861) 2023-04-03 12:58:09 -04:00
repo Use a general Eval function for expressions in templates. (#23927) 2023-04-07 21:25:49 +08:00
shared Introduce GitHub markdown editor, keep EasyMDE as fallback (#23876) 2023-04-03 18:06:57 +08:00
status Remove incorrect HTML self close tag (#23748) 2023-03-27 18:05:51 +02:00
swagger Add activity feeds API (#23494) 2023-04-04 21:35:31 +08:00
user Remove fomantic ".link" selector and styles (#23888) 2023-04-03 20:47:23 -04:00
home.tmpl Improve home page template, fix Sort dropdown menu flash (#23856) 2023-04-01 13:47:54 +08:00
install.tmpl Remove incorrect HTML self close tag (#23748) 2023-03-27 18:05:51 +02:00
post-install.tmpl Remove incorrect HTML self close tag (#23748) 2023-03-27 18:05:51 +02:00