Aiken's build system uses an internal global cache system to avoid
downloading the same packages over and over across projects. However,
prior to this commit, the cache key would be based of the dependency
version which can be either:
- A commit hash
- A branch or tag name
However, in the latter case, it means that the very first time we end
up fetching a dependency will lock its version forever (or until the
cache is cleared). This was inconvenient.
This commit changes that so that we use not only a branch name as
cache key, but additionally, the etag returned by the GitHub API
server. The etag is part of the HTTP headers, so it can be fetched
quickly using a simple HEAD request. It changes whenever the content
behind the endpoint changes -- which happens to be exactly what we
want. With this, we can quickly check whether an upstream package has
been updated and download the latest version should users have
specified a branch name as a version number.
For example, my current cache now looks as follow:
```
/Users/ktorz/Library/Caches/aiken/packages/
├── aiken-lang-stdlib-1cedbe85b7c7e9c4036d63d45cad4ced27b0d50b.zip
├── aiken-lang-stdlib-6b482fa00ec37fe936c93155e8c670f32288a686.zip
├── aiken-lang-stdlib-7ca9e659688ea88e1cfdc439b6c20c4c7fae9985.zip
└── aiken-lang-stdlib-main@04eb45df3c77f6611bbdff842a0e311be2c56390f0fa01f020d69c93ff567fe5.zip
```
This changes allow to use parenthesis `(` `)` to encapsulate
expressions in addition to braces `{` `}` used to define blocks.
The main use-case is for arithmetic and boolean expressions for which
developers are used to using parenthesis. For example:
```
{ 14 + 42 } * 1337
```
can now be written as:
```
( 14 + 42 ) * 1337
```
This may sound straightforward at first but wasn't necessarily trivial
in Aiken given that (a) everything is an expression, (b) whitespaces
do not generally matter and (c) there's no symbol indicating the end
of a 'statement' (because there's no statement).
Thus, we have to properly disambiguate between:
```
let foo = bar(14 + 42)
```
and
```
let foo = bar
(14 + 42)
```
Before this commit, the latter would be interpreted as a function call
and would lead to a somewhat puzzling error. Now, the newline serves
as a delimiting symbol. The trade-off being that for a function call,
the left parenthesis has to be on the same line as the function name
identifier -- which is a fair trade off. So this is still allowed:
```
let foo = bar(
14 + 42
)
```
As there's very little ambiguity about it.
This fixes#236 and would seemingly allow us to get rid of the leading
`#` in front of tuples.
KtorZ