generic-defaults

Generic parameter defaults declared with = ; explicit arguments and inference beat the default.

Syntax: fn f<T = string, size D = 64>(...) — binding order: explicit, then inference, then the declared default

Category#

type

Related#

• generic
• generic-call-type-args
• fn

Examples#

radix/corpus/generic/generic-defaults.fab (canonical · concept)#

Generic parameter defaults declared with = ; explicit arguments and inference beat the default.

# =============================================================================
# generic-defaults — Generic parameter defaults with the = spelling
# =============================================================================
#
# What this teaches:
#   • Generic parameters may declare compile-time defaults: T = string and size D = 64.
#   • Binding order at each call: explicit arguments, then inference, then the declared default.
#   • Related keywords: fn, size
#
# Common mistakes:
#   • Defaults must trail — a parameter without a default may not follow one — and `=` here is a compile-time fact, not the removed vel/coalesce runtime fallback.
#
# See also: generic, generic-call-type-args, fn
# =============================================================================

# generic-defaults — declared = defaults on generic parameters
#
# GRAMMAR:
#   genericParam :← ident genericTypeDefault? | 'size' ident genericSizeDefault?
#   genericTypeDefault :← '=' typeAnnotation
#   genericSizeDefault :← '=' NATURAL
#
# EXPECTED OUTPUT:
#   42
#   media
#
# Explicit and inferred witnesses beat the declared default: pick<int>(42)
# binds T explicitly; pick("media") infers T from the argument. Neither
# call consults `T = string`.
#
# BACKEND: A call whose default actually fires has no call-site MIR
# evidence until specialization consumes the resolved binding vector.
# Defaulted call sites (fallback(), and bounded(1) relying on
# `size D = 64` with D absent from the shapes) are declared here and
# proven by `faber check` and the semantic binder; their execution is
# deferred (whitelist: generic/generic-defaults.fab).

fn pick<T = string>(T x) → T {
    return x
}

fn fallback<T = string>() → string {
    return "salve"
}

fn bounded<size D = 64>(int x) → int {
    return x
}

main {
    print pick<int>(42)
    print pick("media")
}

Expected output:

42
media