CODEWITHSUNDEEP
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Stanko
Stanko

Reputation: 4475

What does the function "return" do in my code?

Type:

data Command a = Command String (a -> IO a)

Functions:

iofunc :: String -> (a -> IO a) -> Command a
iofunc = Command

func :: String -> (a -> a) -> Command a
func s f = iofunc s (return . f)

Can someone explain how (return . f) type checks with (a -> IO a)?

Upvotes: 1

Views: 101

Answers (2)

Sarah
Sarah

Reputation: 6696

I don't know what you mean by "pattern matches" but in this case, return :: a -> IO a. notice that f :: a -> a so by threading the result of that on to return (or composing return with f, if you like) we go from a -> a to a -> IO a

Upvotes: 2

sepp2k
sepp2k

Reputation: 370415

There is no pattern matching going on in your code, so I assume you meant "type checks".

return . f is a function that takes an argument x and evaluates to return (f x). return has the type Monad m => a -> m a and f has the type a -> a. Therefore we know that the type of f x is the same as the type of x and return (f x) then has the type m a where a is the type of x and m is a monad. In other words the type of return . f is Monad m => a -> m a, just like the type of return by itself.

The type required for the second argument to iofunc is a -> IO a. Since IO is a monad, that fits the type Monad m => a -> m a and thus return . f has the proper type to be passed as the second argument to iofunc. Therefore the code type checks.

Upvotes: 6

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