FLIP 288: Simple String Interpolation

cadence/20240923-simple-string-interpolation.md

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+---
+status: draft 
+flip: 288
+authors: Raymond Zhang ([email protected])
+sponsor: Supun Setunga ([email protected]) 
+updated: 2024-09-23
+---
+
+# FLIP 288: Simple String Interpolation
+
+## Objective
+
+This FLIP proposes adding support for simple string interpolation limited to identifiers only (no support for full expressions).
+
+## Motivation
+
+Currently Cadence has no support for string interpolation. It is convenient for developers to be able to inline variables in strings as opposed to the current solution of applying `concat` repeatedly. 
+
+In general many languages support string interpolation for readability and ease-of-use. 
+
+## User Benefit
+
+Developers can avoid repeatedly using `concat` to generate strings which will simplify code and readability.
+
+## Design Proposal
+
+There are many possible implementations two of which are highlighted below. The main constraint is backward compatibility, existing Cadence 1.0 code cannot be affected.
+
+### Python f-string syntax
+Support the following:
+```python
+f"value is {value}"
+```
+This change is backwards compatible as it introduces a new class of strings.
+
+### Swift \\()
+Support the following:
+```swift
+"value is \(value)"
+```
+This change is backwards compatible because `\(` is not currently a valid escape character. 
+
+### Drawbacks
+
+None.
+
+### Alternatives Considered
+
+String interpolation is preferred over a formatting function (such as `printf`) for the following reasons:
+
+- Formatting is error prone (e.g. passing a `String` to `%d`)
+- There are many languages which moved from traditional formatting to string interpolation (e.g. Python)
+
+### Performance Implications
+
+None, non-interpolated strings should not be affected.
+
+### Dependencies
+
+None.
+
+### Engineering Impact
+
+This change should be simple to implement. 
+
+### Best Practices
+
+It may be preferred to use string interpolation over concat once implemented.
+
+### Compatibility
+
+Proposed changes are backwards compatible. 
+
+### User Impact
+
+This is a feature addition, no impact. 
+
+## Related Issues
+
+Extension to support expressions as opposed to just identifiers. Support custom `toString()` functions as well.
+
+## Prior Art
+
+- https://docs.swift.org/swift-book/documentation/the-swift-programming-language/stringsandcharacters/#String-Interpolation
+- https://docs.python.org/3/tutorial/inputoutput.html#formatted-string-literals
+
+## Questions and Discussion
+
+Feel free to discuss on the PR.
+
+Questions:
+- Preferences or concerns on proposed syntax
+- It may be less work to allow expressions versus enforcing identifier only, how much extra testing would be involved?