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import sequtils
var x = @[1, 2, 3]
# This mapIt call will run with preallocation because ``len`` is available.
var y = x.mapIt($(it+10))
doAssert y == @["11", "12", "13"]
type structureWithoutLen = object
a: array[5, int]
iterator items(s: structureWithoutLen): int {.inline.} =
yield s.a[0]
yield s.a[1]
yield s.a[2]
yield s.a[3]
yield s.a[4]
var st: structureWithoutLen
st.a[0] = 0
st.a[1] = 1
st.a[2] = 2
st.a[3] = 3
st.a[4] = 4
# this will run without preallocating the result
# since ``len`` is not available
var r = st.mapIt($(it+10))
doAssert r == @["10", "11", "12", "13", "14"]
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