如何将一系列单子动作应用于 Haskell 中的同一个参数？

Question

In Haskell, is there a way to simplify the following code to remove the repeating of "hello world"? In other words, applying f , g , and h to hello world ?

fn = do
  a <- f "hello world"
  b <- g "hello world"
  c <- h "hello world"
  return (a, b, c)

Answer 1

If f , g and h have the same type, you can simply loop over them:

  [a,b,c] <- forM [f,g,h] ($"hello world")

If they have different types, you can use the fact that functions form a monad, and in particular can be stacked as a monad transformer on top of your current monad:

import Control.Monad.Trans.Reader

fn = (`runReaderT`"hello world") $ do
    a <- ReaderT f
    b <- ReaderT g
    c <- ReaderT h
    return (a, b, c)