Computing all possibilities of replacing one character by another
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    > magicFunction 'l' '_' "hello world"
    ["he_lo world", "hel_o world", "hello wor_d"]

Is there such a magic function in the standard Prelude or can it be composed easily with other functions?

And no, this isn't homework, but still, please don't spend too much time hand-rolling your own complicated solution, I'd rather do that myself than waste your time ;) Just asking if it's in the standard.