Question

The error must be so tiny in these few lines that I will not get it myselft.

Here is my code:

askName = do
  putStr "Type your name: "
  name <- getLine
  return name

sayHelloTo name = do
  when (null name) (name <- askName)

Apparently it gives an error:

1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:9:30: parse error on input `<-'
Failed, modules loaded: none.

Any suggestion?

Edit 1.

Same if I write:

sayHelloTo name = do
  when (null name) (name2 <- askName)

Answer 1

The name2 <- syntax is part of do-notation and can only be used inside a do block. It is also not a variable assignment – under the hood it is just creating a callback function that has name2 as a parameter

That is, the following code:

do
    name2 <- monadicOp
    ...things...

desugars into

monadicOp >>= (\name2 -> ...things... )

I hope this helps make it clear that you don’t assign things or mutate them in Haskell.

anyway, to solve your particular problem, whay you could do is just use if-then-else (something that is actually equivalent to the ?: ternary oerator) and return the appropriate value. For example, the following function uses recursion to ask for a name again and again until it is happy with the result

getNonEmptyName :: IO String
getNonEmptyName = do
    name <- getName
    if null name --note: indenting if statements in do blocks is tricky
       then getNonEmptyName
       else (return name)

or, without the do notation sugar:

getNonEmptyName = getName >>= (\name -> if (null name) then getNonEmptyName else (return name) )

This might be a bit moe different the what you are used to, but I guess you should be able to clear thing sup after you get how this is working. Basically getNonEmptyName is of type IO String, meaning it is an IO actioin IO action that yields a String when it is run. The if-then-else part is supposed to evaluate to an IO String value as well, since its value will be the return value of getNonEmptyName. This all works all right since in the first we do a recursive call to getNonEmptyName (and that gives an IP String value as desired) and in the else branch we promote a regular string value (name) to an IO String using the return function.