Feat: The Mamba-style indexing of collections

[JSAbrahams]

Summary

Unlike C-style languages, we index collections using round brackets regardless of collection type.
E.g. we don't distinguish between lists and maps.
We had wanted to implement this for some time, but now we actually implemented this.

This feature essentially fully leverages the type checker to detect if we are indexing a collection, and passes this information to the (Python) backend.
This means that collection indexing is decoupled from our syntax.
Concretely, this means that mamba style indexing ( and ) is still desugared to Pythons [] C-style indexing.

def my_collection = [1, 2, 3]
print(my_collection(2)) # prints 3

Becomes

my_collection = [1, 2, 3]
print(my_collection[2]) # prints 3

In general, we also try to expand the code example section.
The README elaborates a bit more on why we opted for this perhaps unusual syntax which deviates from the C-style which dominates all languages.

The advantage of this decoupling is that it can work either way. If in future we opt to change our syntax (back/again), we can leave our backend untouched. We mainly need to change the type checker then to reject function calls and augment the grammar (if necessary) for indexes.

Added Tests

• Tested for regressions.
• And also tested the list indexing results in the Python code we expect.

[codecov]

Codecov Report

❌ Patch coverage is 86.63102% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.33%. Comparing base (8155000) to head (e3edf1c).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/check/constrain/constraint/expected.rs	0.00%	9 Missing ⚠️
src/check/constrain/generate/call.rs	86.15%	9 Missing ⚠️
src/check/constrain/unify/function.rs	92.04%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #498      +/-   ##
===========================================
- Coverage    92.50%   92.33%   -0.17%     
===========================================
  Files          113      113              
  Lines        15709    15911     +202     
===========================================
+ Hits         14531    14692     +161     
- Misses        1178     1219      +41     

Files with missing lines	Coverage Δ
src/backend/cranelift/convert/call.rs	90.07% <100.00%> (+0.15%)	⬆️
src/backend/python/convert/call.rs	81.25% <100.00%> (+6.25%)	⬆️
src/backend/python/convert/mod.rs	95.50% <100.00%> (+0.02%)	⬆️
src/check/ast/mod.rs	100.00% <100.00%> (ø)
src/check/ast/node.rs	99.16% <ø> (ø)
src/check/constrain/unify/expression.rs	100.00% <100.00%> (ø)
src/check/constrain/unify/finished.rs	100.00% <ø> (ø)
src/check/constrain/unify/ty.rs	95.96% <100.00%> (+0.13%)	⬆️
src/check/context/function/python.rs	96.00% <ø> (ø)
src/check/constrain/unify/function.rs	82.20% <92.04%> (-2.39%)	⬇️
... and 2 more

... and 4 files with indirect coverage changes

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