Type System

Type System

Scratch is dynamically typed — any variable can hold a number or string at runtime. C is statically typed. The type inference pass resolves this gap by scanning how each variable is used and picking a type.

Types

class ScratchType(Enum):
    UNKNOWN = auto()  # not yet resolved
    LONG = auto()     # C long — 64-bit integer
    STRING = auto()   # C char[256] — fixed-size buffer

Floats are not supported. Scratch uses IEEE 754 doubles internally, but for the text/systems programs this tool targets, long is sufficient. See Known Limitations.

Two-pass algorithm

Pass 1 — Assignment scanning

_scan_statements() walks every statement and looks at what values are assigned to each variable:

• SetVariable where the value is arithmetic → LONG
• SetVariable where the value is scratch_join() → STRING
• SetVariable where the value is a non-numeric string literal → STRING
• SetVariable where the value is scratch_contains() → LONG
• ChangeVariable (always numeric) → LONG

Conflict resolution: if a variable is assigned both numeric and string values, STRING wins. A char[256] can hold "42" just fine; a long can't hold "hello".

Pass 2 — Context propagation

_propagate_statements() looks at where variables are used:

• Variable used in operator_add → pin as LONG
• Variable used in scratch_join → pin as STRING
• Variable used in a comparison → pin as LONG

This resolves variables that Pass 1 couldn't classify (e.g., a variable that's only read, never assigned).

Final default

After both passes, any variable still UNKNOWN defaults to LONG. This matches Scratch's behavior for uninitialized variables (they default to 0).

How types affect code generation

Aspect	LONG	STRING
Declaration	long x = 0;	char x[256] = "";
Assignment	x = value;	snprintf(x, sizeof(x), "%s", value);
Say/print	printf("%ld\n", x)	printf("%s\n", x)
Used in arithmetic	x (direct)	atol(x)
Used in join/say	scratch_ltoa(x)	x (direct)

Adding type awareness for new functions

When you add a new CallExpr function (like scratch_contains), you need to register its return type in three places:

# 1. type_inference.py → _classify_expression()
if expr.func in ("scratch_strlen", "scratch_contains"):
    return ScratchType.LONG

# 2. type_inference.py → _propagate_expr_context()
elif expr.func == "scratch_contains":
    for arg in expr.args:
        _propagate_expr_context(arg, ScratchType.STRING, variables)

# 3. codegen/base.py → _expr_type()
if expr.func in ("scratch_strlen", "scratch_contains"):
    return ScratchType.LONG

Missing any of these causes incorrect type inference or wrong format specifiers in the output.