sum-empty-float

Returns 0.0, not 0, for an empty float list.

Syntax: <list<float>>.sum()

Category#

collection

Related#

• prima
• ultima
• list

Examples#

radix/corpus/lista/sum-empty-float.fab (canonical · existing-home)#

Returns 0.0, not 0, for an empty float list.

# =============================================================================
# sum — an empty float list sums to the float zero.
# =============================================================================
#
# What this teaches:
#   • `sum()` is the fold that starts at the element type's additive identity,
#     so an empty `list<float>` sums to the float zero `0.0` — never to the
#     integer `0`, and never to a trap.
#   • Empty input is not a special case at the call site: the same `sum()` works
#     on an empty list, a populated list, and a list bound inside a function that
#     returns the fold.
#
# Common mistakes:
#   • Assuming an empty `sum()` has no answer and guarding it — it has one, and
#     the guard hides the type the fold carries.
#   • Assuming the empty result is the integer `0`; it is `0.0`, the float
#     additive identity of the element type.
#
# See also: prima, ultima, list
# =============================================================================

# list<T>.sum() additive identity on the empty list
#
# e.sum()                  -- sum of the elements, element type's zero when empty
#
# GRAMMAR:
#   listDecl :← ('let' | 'mut') 'list<' type '>' ident '←' expr
#   listSum  :← expr '.' 'sum' '(' ')'
#
# EXPECTED OUTPUT:
#   0.0
#   0.0
#   true
#   4.0
#   0.0
#
# BACKEND: runner-pinned stdout (sum-empty-float.expected).

main {
    let list<float> e ← empty
    print e.sum()
    let float z ← e.sum()
    print z
    print e.sum() ≡ 0.0
    let list<float> f ← [1.5, 2.5]
    print f.sum()
    print sum_return()
}

fn sum_return() → float {
    let list<float> none_yet ← empty
    return none_yet.sum()
}

Expected output:

0.0
0.0
true
4.0
0.0