haskell - Why doesn't `putStrLn getLine` work? -

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i'm complete newbie on haskell. haskell script ghci,

prelude> let = putstrln getline

makes error this.

<interactive>:1:17:     couldn't match expected type `string'            against inferred type `io string'     in first argument of `putstrln', namely `getline'     in expression: putstrln getline     in definition of `a': = putstrln getline prelude>

why doesn't work , how can print input stdin?

putstrln :: string -> io () getline :: io string

the types not match. getline io action, , putstrln takes plain string.

what need bind line inside io monad in order pass putstrln. following equivalent:

a = line <- getline        putstrln line  = getline >>= \line -> putstrln line  = getline >>= putstrln

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