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I/O Builtins

Clausal provides built-in predicates for formatted output and term-to-string conversion. The .clausal file format also supports f-string interpolation for string construction.

Output Predicates

Builtin Arity Description
Write 1 Write a term to stdout (no newline)
Writeln 1 Write a term to stdout followed by a newline
PrintTerm 1 Write a term using its string representation
Nl 0 Write a newline
Tab 1 Write N spaces

Examples

greet(NAME) <- (Write("Hello, "), Write(NAME), Nl())

show_all(XS) <- (
    In(X, XS),
    Writeln(X)
)

indented(X) <- (Tab(4), Writeln(X))

String Conversion

Builtin Arity Description
WriteToString 2 WriteToString(Term, String) — unify String with the Write representation
TermToString 2 TermToString(Term, String) — unify String with the str() representation 
format_pair(K, V, S) <- WriteToString(K - V, S)

F-String Support

In .clausal files, f-strings are supported for string construction. Logic variables are automatically dereferenced before interpolation:

describe(NAME, AGE, S) <- (
    S is f"Name: {NAME}, Age: {AGE}"
)

This uses FStringThunk / FStringPart for deferred evaluation — the f-string is evaluated at search time after variables are bound.

Var Display

Logic variables have __str__ and __format__ methods (in the C extension) that auto-deref for display:

  • Bound var: displays the bound value
  • Unbound var: displays _VarN (unique ID)

This means f"{X}" and Write(X) show the value if bound, or a placeholder if unbound.

Test coverage

Tests are in tests/test_io.py (43 tests).

  • Var display: __str__, __format__, bound/unbound, nested
  • Write/Writeln/PrintTerm: atoms, numbers, strings, compounds, lists, vars
  • Nl/Tab: output formatting
  • WriteToString/TermToString: term conversion to string
  • F-string integration: variable interpolation, multiple vars, expressions

See also: Python Integration — using ++() escape for Python calls inside logic goals.