Comment by tylerhou
Comment by tylerhou 14 hours ago
Yes.
Comment by tylerhou 14 hours ago
Yes.
A Monad is a _super_ generic interface that can be implemented for a whole bunch of structures/types. When people talk about "monads", they are usually referring to a specific instance. In this case, the Reader monad is a specific instance that is roughly equivalent to functions that take an argument of a particular type and return a result of any type. That is, any function that looks like this (r -> a) where `r` is fixed to some type, and `a` can be anything.
Functions of that form can actually implement the Monad interface, and can make use of Haskells syntax support for them.
One common use-case for the reader monad pattern is to ship around an interface type (say, a struct with a bunch of functions or other data in it). So, what people are saying here is that passing around a the `Io` type as a function argument is just the "reader monad" pattern in Haskell.
And, if you hand-wave a bit, this is actually how Haskell's IO is implemented. There is a RealWorld type, which with a bit of hand waving, seems to pretty much be your `Io` type.
Now, the details of passing around that RealWorld type is hidden in Haskell behind the IO type, So, you don't see the `RealWorld` argument passed into the `putStrLn` function. Instead, the `putStrLn` function is of type `String -> IO ()`. But you can, think of `IO ()` as being equivalent to `RealWorld -> ()`, and if you substitute that in you see the `String -> RealWorld -> ()` type that is similar to how it appears you are doing it in Zig.
So, you can see that Zig's Io type is not the reader monad, but the pattern of having functions take it as an argument is.
Hopefully that helps.
---
Due to Haskell's laziness, IO isn't actually the reader monad, but actually more closely related to the state monad, but in a strict language that wouldn't be required.
A reader is just an interface that allows you to build up a computation that will eventually take an environment as a parameter and return a value.
Here's the magic:
newtype Reader env a = Reader { runReader :: env -> a }
ask = Reader $ \x -> x
instance Functor (Reader env) where
fmap f (Reader g) = Reader $ \x -> f (g x)
instance Applicative (Reader env) where
pure x = Reader (\_ -> x)
ff <*> fx = Reader $ \x -> (runReader ff x) (runReader fx x)
instance Monad (Reader env) where
(Reader f) >>= g = Reader $ \x -> runReader (g (f x)) x
Might be used like this:
calc :: Reader String Int
calc = do
input <- ask
pure $ length input
test :: Int
test = runReader calc "Test"
-- returns: 4
https://stackoverflow.com/questions/14178889/what-is-the-pur...
Edit: Added Applicative instance so code runs on modern Haskell. Please critique! Also added example.
Here's a minimal python translation of the important bits:
class Reader:
def __init__(self, func):
self.run = func
def pure(x):
return Reader(lambda _: x)
def bind(self, f):
return Reader(lambda env: f(self.run(env)).run(env))
ask = Reader(lambda env: env)
def calc():
return ask.bind(lambda input_str:
Reader.pure(len(input_str)))
test = calc().run("test")
print(test)
-- this:
calc :: Reader String Int
calc = do
input <- ask
pure $ length input
-- translates to:
calc' :: Reader String Int
calc' = ask >>= (\input -> pure $ length input)I see I’ve been beaten to the punch, but I’ll post my try anyway.
Your comment about IO handled by an external system In response to a comment about the more general concept of a monad is what they are, somewhat abruptly referring to in the above two comments.
The IO monad in Haskell is somewhat ‘magical’ in that it encapsulates a particular monad instance that encodes computational actions which Haskell defers to an external system to execute. Haskell chose to encode this using a monadic structure.
To be a bit more particular:
The Reader monad is the Haskell Monad instance for what can generically be called an ‘environment’ monad. It is the pattern of using monadic structure to encapsulate the idea of a calling context and then taking functions that do not take a Context variable and using the encapsulating Monad to provide the context for usage within that function that needs it.
Based on your streams in the new system I don’t see a monad, mostly because the Reader instance would basically pipe the IO parameter through functions for you and Zig requires explicit passage of the IO (unless you set a global variable as IO but that’s not a monad, that’s just global state) to each function that uses it.
From my perspective Zig’s IO looks to be more akin to a passed effect token outside the type system ‘proper’ that remains compile time checked by special case.
Reader monad is a fancy way of saying ‘have the ability to read some constant value throughout the computation’. So here they mean the io value that is passed between functions.
Well I don't think that fits at all. In Zig, an Io instance is an interface, passed as a parameter. You can draw some connections between what Zig is doing and what Haskell is doing but it's not a monad. It's plain old interfaces and parameters, just like Allocator.
Passing an interface as a parameter is a monad. (Io -> _) is an instance of Monad in Haskell.
Haskell just has syntax to make using (any) monad much nicer. In this case, it let's you elide the `Io` parameter in the syntax if you are just going to be passing the same Io to a bunch of other functions. But it still is there.
Let's see if I can do it without going too far off the deep end. I think your description of the _IO type_ as "a description of how to carry out I/O that is performed by a separate system" is quite fair. But that is a property of the IO type, not of monads. A monad in programming is often thought of as a type constructor M (that takes and returns a type), along with some functions that satisfy certain conditions (called the "monad laws").
The `IO` type is a type constructor of one argument (a type), and returns a type: we say that it has kind `Type -> Type`, using the word "kind" to mean something like "the 'type' of a type". (I would also think of the Zig function `std.ArrayList` as a type constructor, in case that's correct and useful to you.) `IO String` is the type of a potentially side-effecting computation that produces a `String`, which can be fed to other `IO`-using functions. `readLine` is an example of a value that has this type.
The Haskell function arrow `(->)` is also a type constructor, but of two arguments. If you provide `(->)` with two types `a` and `b`, you get the type of functions from `a` to `b`:
`(->)` has kind `Type -> Type -> Type`.
`(->) Char` has kind `Type -> Type`.
`(->) Char Bool` has kind `Type`. It is more often written `Char -> Bool`. `isUpper` is an example of a value that has this type.
The partially-applied type constructor `(->) r`, read as the "type constructor for functions that accept `r`", is of the same kind as `IO`: `Type -> Type`. It also turns out that you can implement the functions required by the monad interface for `(->) r` in a way that satisfies the necessary conditions to call it a monad, and this is often called the "reader monad". Using the monad interface with this type constructor results in code that "automatically" passes a value to the first argument of functions being used in the computation. This sometimes gets used to pass around a configuration structure between a number of functions, without having to write that plumbing by hand. Using the monad interface with the `IO` type results in the construction of larger side-effecting computations. There are many other monads, and the payoff of naming the "monad" concept in a language like Haskell is that you can write functions which work over values in _any_ monad, regardless of which specific one it is.
I tried to keep this brief-ish but I wasn't sure which parts needed explanation, and I didn't want to pull on all the threads and make a giant essay that nobody will read. I hope it's useful to you. If you want clarification, please let me know.
This is pretty concise, but is still really technical. That aside, I think the actual bone of contention is that Zig’s IO is not a Reader-esque structure. The talks and articles I’ve read indicate that function needing the IO ‘context’ must be passed said context as an argument. Excepting using a global variable to make it available everywhere, but as I said in a sibling comment, that’s just global state not a monad.
In a manner of speaking, Zig created the IO monad without the monad (which is basically just an effect token disconnected from the type system). Zig’s new mechanism take a large chunk of ‘side-effects’ and encapsulates them in a distinct and unique interface. This allows for a similar segregation of ‘pure’ and ‘side-effecting’ computations that logically unlined Haskell’s usage of IO. Zig however lacks the language/type system level support for syntactically and semantically using IO as an inescapable Monad instance. So, while the side effects are segregated via the IO parameter ‘token’ requirement they are still computed as with all Zig code. Finally, because Zig’s IO is not a special case of Monad there is no restriction on taking IO requiring results of a function and using them as ‘pure’ values.
Can you explain for those of us less familiar with Haskell (and monads in general)?