feat: .as_T cast methods

[aaron-ang]

Changes Made

Use decorator to record datatypes and generate methods at runtime. Also included function headers for intellisense.

Related Issues

Closes #4014.

[greptile-apps]

Greptile Overview

Greptile Summary

This PR adds convenience .as_T() cast methods to the Expression class, providing syntactic sugar for .cast(DataType.T()). The implementation uses a decorator pattern to automatically register all DataType constructor methods at module load time, then dynamically generates corresponding as_* methods on the Expression class.

Key Changes:

• Added @datatype_constructor decorator in daft/datatype.py to track all DataType constructor methods
• Generated .as_* methods for all 30+ data types (e.g., .as_string(), .as_int64(), .as_timestamp())
• Included type stubs in TYPE_CHECKING block for IDE autocomplete and type checking
• Added comprehensive tests verifying the generated methods produce identical results to manual cast() calls

The implementation is clean and maintainable - adding new DataTypes in the future will automatically generate corresponding .as_* methods without additional code.

Confidence Score: 5/5

• This PR is safe to merge with no identified issues
• The implementation is elegant, well-tested, and follows good software engineering practices using decorators and runtime code generation
• No files require special attention

Important Files Changed

Filename	Overview
daft/datatype.py	Adds @datatype_constructor decorator to register DataType constructor methods and enables dynamic method generation
daft/expressions/expressions.py	Adds as_* cast methods to Expression class with type stubs for IDE support and runtime generation
tests/expressions/test_expressions.py	Adds tests verifying as_* methods produce identical results to cast() for simple and parameterized types

Sequence Diagram

sequenceDiagram
    participant User
    participant Expression
    participant DataType
    participant PyDataType
    
    Note over DataType: Module Load Time
    DataType->>DataType: Apply @datatype_constructor to methods
    DataType->>DataType: Populate _DATATYPE_CONSTRUCTOR_SET
    
    Note over Expression: Module Load Time
    Expression->>DataType: Call _constructor_names()
    DataType-->>Expression: Return sorted tuple of constructor names
    Expression->>Expression: Generate as_* methods via setattr
    Expression->>Expression: Create _make_expr_as_dtype_method for each
    
    Note over User: Runtime Usage
    User->>Expression: col("x").as_string()
    Expression->>DataType: DataType.string()
    DataType->>PyDataType: Create PyDataType
    PyDataType-->>DataType: Return PyDataType instance
    DataType-->>Expression: Return DataType wrapper
    Expression->>Expression: self.cast(dtype)
    Expression-->>User: Return casted Expression

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[greptile-apps]

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[codecov]

Codecov Report

❌ Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 43.44%. Comparing base (aa8add2) to head (59f5e00).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
daft/datatype.py	97.61%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6100       +/-   ##
===========================================
- Coverage   72.91%   43.44%   -29.48%     
===========================================
  Files         973      909       -64     
  Lines      126196   112794    -13402     
===========================================
- Hits        92016    49001    -43015     
- Misses      34180    63793    +29613     

Files with missing lines	Coverage Δ
daft/expressions/expressions.py	95.19% <100.00%> (+0.07%)	⬆️
daft/datatype.py	93.61% <97.61%> (+0.25%)	⬆️

... and 653 files with indirect coverage changes

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[universalmind303]

honestly, i'm not a huge fan of this functionality (as .cast does the same thing), but will defer to others on if we want to merge it or not.