John Wicket wrote:
Sorry, I was actually trying to use this as an example for something
more complicated I am trying to do. In this example, why would the
inferred type be "IO ()"
aa :: String -> String
aa instr = do
putStrLn "abc"
putStrLn "abc"
return "Az"
Couldn't match expected type `[t]' against inferred type `IO ()'
In the expression: putStrLn "abc"
In a 'do' expression: putStrLn "abc"
In the expression:
do putStrLn "abc"
putStrLn "abc"
return "Az"
If you change that type signature to
aa :: String -> IO String
then it will work.
Any code that does any I/O must have a result type "IO blah". If the
code returns no useful information, it will be "IO ()". If, like above,
it returns a string, it will be "IO String". And so on.
(I must say, that error message doesn't make it terribly clear what the
problem is...)
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