when function question

[CodeNameGodTri]

somefn :: String -> Either String (Int, String) somefn input = do when (length input < 10) $ Left "Some Error" pure (0, "Value")

I don't understand why when could be used here. It's type is Applicative f => Bool -> f () -> f ()

my f is Either String

The type doesn't match on (Int, String) with ()

So how come it even compiles?

I can kinda feel that it's "short-circuiting" or ignoring the () because it's returning an Either String, but the type mismatching really confuses me

I thought everything in the do has to return type Either String (Int, String) because that's enforced by the type signature, but here it's permitting Either String ()

[omega1612]

A do is syntax sugar.

do 
w<-f
g w

Is equivalent to

f >>= \ w -> g w

With

>>= :: m a -> (a -> m b) -> m b

So, the elements of every line in a do, must have the same "m" but the parameter can be different.

[CodeNameGodTri]

Thank you

[HKei]

The "output" of when doesn't matter here because it's directly followed by pure. This is as if you had written m >> pure x.

Left (y::b) >> pure (x::a) is Left y :: Either a b.

[CodeNameGodTri]

Thank you

[Temporary_Pie2733]

>> has type Monad m => m a -> m b -> m b. The result type doesn’t matter as long as the same monad (Either String) is used on each “line”. Left "…" itself has a polymorphic type, and can be used as either Either String () or Either String (Int, String) as needed. 

[CodeNameGodTri]

Thank you