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Typed bindings and checked conversions (v0.0.5-alpha.2)

This practical lesson covers two capabilities shipped in the published v0.0.5-alpha.2 prerelease on November 2, 2026: optional type annotations in bind / var declarations and explicit, checked numeric conversions. The source is pinned to compiler commit e4db8e191cd7ea623123cae7348450ca6c95b170. It is a release-specific addition; the older variables and types references retain their independent alpha.1 evidence.

1. What is a type annotation?

A binding stores a value under a name. With bind, reassignment is forbidden; with var, the binding may be reassigned to another value of the same type. Scarp ordinarily infers the type from the initializer. Since the published alpha.2 release, you can also write name: type before = to state the intended type explicitly.

An annotation is a check, not a cast. The initializer must already have the declared type. It does not make a value mutable or change its representation. An initializer is still mandatory, and bind remains immutable even when annotated.

Published program: typed bindings

Save this exact file as typed_declarations.srp. It is copied from the tagged official compiler example:

bind samples: vector = [1.0, 2.0, 3.0];
var total: float = sum(samples);
total += 0.5;
bind grid: tile = tile(samples, 1, 3);
bind ready: boolean = width(grid) == 3;
bind label: string = "typed";
print(label, ready, total);

Expected stdout from the tagged compiler:

typed true 6.5

Read it from top to bottom:

  • samples: vector declares an immutable floating-point vector.
  • total: float declares a mutable finite float initialized with the result of sum(samples), which is 6.0.
  • total += 0.5 changes that float to 6.5. It does not change samples.
  • grid: tile stores a tiled numerical value, while ready: boolean checks that its width is three.
  • label: string keeps the text "typed". print writes all three selected values on one line.

Together, the two published examples demonstrate integer, float, boolean, string, vector and tile type annotations; it does not introduce classes, user-defined structures, a generic type system, or a general-purpose list.

2. Why are conversions explicit?

integer and float are different types. Scarp does not silently turn an integer into a float in arithmetic or during declaration. The published alpha.2 release provides two pure, checked built-ins:

Built-in Argument and result Important rule
to_float(value) integer → float The integer must be exactly representable as a finite binary64 float.
to_integer(value) float → integer The float must be mathematically integral and within the signed 64-bit integer range.

A fractional float is rejected, not rounded or truncated. If the integer cannot be exactly expressed as a binary64 number, to_float also fails rather than silently losing precision. Both failures use the compiler's E0044 diagnostic; this is not an arbitrary unchecked cast.

Published program: numeric conversions

Save this exact tagged compiler source as scalar_conversions.srp:

bind count: integer = 42;
bind scaled: float = to_float(count) / 4.0;
bind exact: integer = to_integer(12.0);
print(scaled, exact, to_integer(-0.0));

Expected stdout:

10.5 12 0

Explanation: to_float(count) converts the integer 42 to the float 42.0; dividing by 4.0 gives 10.5. to_integer(12.0) becomes 12. Negative floating-point zero (-0.0) converts to integer zero (0). The source uses annotations to make both intended results clear.

3. Run both programs using the published compiler

Save the two .srp files in the same working directory and make sure the alpha.2 executable is on your PATH. Run:

scarp --version
scarp check typed_declarations.srp
scarp run typed_declarations.srp
scarp check scalar_conversions.srp
scarp run scalar_conversions.srp

First confirm that scarp --version reports 0.0.5-alpha.2. The historical check command suppresses user output but can evaluate selected runtime paths; it is not the same as the static-only verify available in alpha.3. run executes each program and prints the expected output shown above.

If you run an extracted binary without installing it globally, use ./scarp on Linux/macOS or .\scarp.exe in Windows PowerShell instead of scarp. File paths are resolved from your terminal's current directory.

4. Common mistakes and version-specific limits

Mistake Why it matters
Expecting bind number: float = 42; to cast automatically An annotation checks the initializer's type; it does not convert integer to float.
Treating to_integer as a truncation operation Fractional floats are rejected, not truncated.
Assuming var allows a new type on every assignment Mutations preserve the originally declared type.
Using scarp verify on an alpha.2 executable That command belongs to the published alpha.3 CLI; use version-appropriate commands.
Expecting a vector to store integers The vector type holds finite floats; the separate alpha.2 integers collection is described in collections.

Try predicting the output before running the two files. Then change total += 0.5 to a different float increment in your own copy and predict the new result. Use scarp check to observe any diagnostics. Keep the original examples unchanged when checking against the published stdout fixtures.

Continue learning

Read variables and lexical scope, data types, the 12-case published alpha.2 feature index, and alpha.3 getting started for the newer CLI.

Evidence: The complete sources and stdout above match tagged compiler fixtures, copied in conformance/alpha2/ and executed by the release-specific compiler CI job. The docs build additionally verifies that both language versions reproduce these fixtures exactly.

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