▁prem ise ▁that ▁( refer ring ▁to ▁a ▁value ) ▁" if ▁it ▁wal ks ▁like ▁a ▁du ck , ▁and ▁qu acks ▁like ▁a ▁du ck , ▁then ▁it ▁is ▁a ▁du ck " ▁( this ▁is ▁a ▁reference ▁to ▁the ▁du ck ▁test ▁that ▁is ▁attributed ▁to ▁James ▁Whit comb ▁R iley ). ▁The ▁term ▁may ▁have ▁been ▁co ined ▁ ▁by ▁Alex ▁Mart elli ▁in ▁a ▁ 2 0 0 0 ▁message ▁to ▁the ▁comp . lang . python ▁news group ▁( see ▁Python ). ▁ ▁While ▁one ▁controlled ▁experiment ▁showed ▁an ▁increase ▁in ▁developer ▁product ivity ▁for ▁du ck ▁typing ▁in ▁single ▁developer ▁projects , ▁other ▁controlled ▁experiments ▁on ▁API ▁us ability ▁show ▁the ▁opposite . ▁ ▁Special ized ▁type ▁systems ▁ ▁Many ▁type ▁systems ▁have ▁been ▁created ▁that ▁are ▁special ized ▁for ▁use ▁in ▁certain ▁environments ▁with ▁certain ▁types ▁of ▁data , ▁or ▁for ▁out - of - band ▁static ▁program ▁analysis . ▁Fre qu ently , ▁these ▁are ▁based ▁on ▁ideas ▁from ▁formal ▁type ▁theory ▁and ▁are ▁only ▁available ▁as ▁part ▁of ▁prototype ▁research ▁systems . ▁ ▁The ▁following ▁table ▁gives ▁an ▁over view ▁over ▁type ▁the or etic ▁concepts ▁that ▁are ▁used ▁in ▁special ized ▁type ▁systems . ▁The ▁names ▁ ▁range ▁over ▁terms ▁and ▁the ▁names ▁ ▁range ▁over ▁types . ▁The ▁notation ▁ ▁( resp . ▁) ▁describes ▁the ▁type ▁which ▁results ▁from ▁replacing ▁all ▁occurr ences ▁of ▁the ▁type ▁variable ▁ ▁( resp . ▁term ▁variable ▁) ▁in ▁ ▁by ▁the ▁type ▁