Is there an escape from MonadState+MonadIO+MonadError monad stack?

newer
code-as-config, run-time checks...

older
Re: [Haskell-cafe] Haskell is a...

Ömer Sinan Ağacan

6 Apr 2013 6 Apr '13

5:22 p.m.

Hi, I'm a hobbyist Haskell programmer and my use of Haskell is mostly consists of writing interpreters, simple virtual machines, and type checkers. One thing I'm not happy about my Haskell programs is, almost all of my programs have a monad transformer stack consisting MonadError, MonadIO and MonadState. Let's say I'm writing an interpreter, I certainly need MonadIO to interpret object language's IO functions: printing, reading from file etc. I also need MonadState for, well, states(dynamic environment etc.). MonadError is also required because most computations can fail(unbound variable error, type mismatch etc. it can be also used for implementing exceptions in object language) Same applies for my other applications as well. Type checkers require MonadError(unification error etc.), MonadState(to keep substitutions). MonadIO is not required in this case. But it also required for virtual machines(IOVector for memory -- ST vectors also work, but IO is still required for other stuff - printing, display etc.) I know these are mostly related with my application area, but I still wanted to write this because I may be missing something, or simply doing things wrong. Advices from experienced Haskell programmers would be appreciated, Cheers, Ömer

Show replies by date

Kim-Ee Yeoh

6 Apr 6 Apr

5:40 p.m.

New subject: Is there an escape from MonadState+MonadIO+MonadError monad stack?

On Sun, Apr 7, 2013 at 4:22 AM, Ömer Sinan Ağacan wrote:

...

I'm a hobbyist Haskell programmer and my use of Haskell is mostly consists of writing interpreters, simple virtual machines, and type checkers.

One thing I'm not happy about my Haskell programs is, almost all of my programs have a monad transformer stack consisting MonadError, MonadIO and MonadState.

Welcome! Hobbyist Haskellers writing VMs and type checkers are a critical part of the community and what sets us apart. Not as well-known as it should be is the fact that GHC doesn't make much use of monad transformers. Have you taken a look at the sources? That might provide ideas on future ways of structuring your experiments. Also, what precisely are the infelicities with monad transformers in your code? Depth of stack? Forced type annotation? Syntax inflation due to extra lift* functions? Monad transformers provide an abstraction which may not be necessary for some apps. But it's easy to write something and then suddenly, the need for generalization kicks in. -- Kim-Ee

Ömer Sinan Ağacan

6:03 p.m.

New subject: Is there an escape from MonadState+MonadIO+MonadError monad stack?

...

Not as well-known as it should be is the fact that GHC doesn't make much use of monad transformers. Have you taken a look at the sources? That might provide ideas on future ways of structuring your experiments.

That's interesting, thanks! Do you have any recommendations about which file to start reading? AFAIK, GHC is _huge_. Ömer

Kim-Ee Yeoh

7 Apr 7 Apr

6:39 a.m.

New subject: Is there an escape from MonadState+MonadIO+MonadError monad stack?

On Sun, Apr 7, 2013 at 5:03 AM, Ömer Sinan Ağacan wrote:

...

That's interesting, thanks! Do you have any recommendations about which file to start reading? AFAIK, GHC is _huge_.

Without a discussion of your interests, it's hard to say. Certainly, I'd set up the reading environment, namely an editor that can traverse from usage to point of definition and back. An interesting idea that occurred to me is to start with the file with the largest comments to code ratio. -- Kim-Ee

Roman Cheplyaka

6 Apr 6 Apr

5:42 p.m.

New subject: Is there an escape from MonadState+MonadIO+MonadError monad stack?

Hi Ömer, You forgot to mention what your problem is. :) What you describe sounds reasonable. Why do you want to "escape"? Roman * Ömer Sinan Ağacan [2013-04-07 00:22:58+0300]

...

Hi,

I'm a hobbyist Haskell programmer and my use of Haskell is mostly consists of writing interpreters, simple virtual machines, and type checkers.

One thing I'm not happy about my Haskell programs is, almost all of my programs have a monad transformer stack consisting MonadError, MonadIO and MonadState.

Let's say I'm writing an interpreter, I certainly need MonadIO to interpret object language's IO functions: printing, reading from file etc. I also need MonadState for, well, states(dynamic environment etc.). MonadError is also required because most computations can fail(unbound variable error, type mismatch etc. it can be also used for implementing exceptions in object language)

Same applies for my other applications as well. Type checkers require MonadError(unification error etc.), MonadState(to keep substitutions). MonadIO is not required in this case. But it also required for virtual machines(IOVector for memory -- ST vectors also work, but IO is still required for other stuff - printing, display etc.)

I know these are mostly related with my application area, but I still wanted to write this because I may be missing something, or simply doing things wrong.

Advices from experienced Haskell programmers would be appreciated,

Cheers, Ömer

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Ömer Sinan Ağacan

6:02 p.m.

New subject: Is there an escape from MonadState+MonadIO+MonadError monad stack?

...

You forgot to mention what your problem is. :)

What you describe sounds reasonable. Why do you want to "escape"?

It's not really a problem, I'm not happy with this design because to me it was like I'm missing the point of using a 'functional' language. For instance, in most parts of my code I can actually do IO. On the other hand, I know it's mostly because of my application area and nothing to do with Haskell. Let's say I just wanted to know ideas of experienced Haskell programmers :-) Ömer --- Ömer Sinan Ağacan http://osa1.net/ 2013/4/7 Roman Cheplyaka :

...

Hi Ömer,

You forgot to mention what your problem is. :)

What you describe sounds reasonable. Why do you want to "escape"?

Roman

* Ömer Sinan Ağacan [2013-04-07 00:22:58+0300]

...
Hi,

I'm a hobbyist Haskell programmer and my use of Haskell is mostly consists of writing interpreters, simple virtual machines, and type checkers.

One thing I'm not happy about my Haskell programs is, almost all of my programs have a monad transformer stack consisting MonadError, MonadIO and MonadState.

Let's say I'm writing an interpreter, I certainly need MonadIO to interpret object language's IO functions: printing, reading from file etc. I also need MonadState for, well, states(dynamic environment etc.). MonadError is also required because most computations can fail(unbound variable error, type mismatch etc. it can be also used for implementing exceptions in object language)

Same applies for my other applications as well. Type checkers require MonadError(unification error etc.), MonadState(to keep substitutions). MonadIO is not required in this case. But it also required for virtual machines(IOVector for memory -- ST vectors also work, but IO is still required for other stuff - printing, display etc.)

I know these are mostly related with my application area, but I still wanted to write this because I may be missing something, or simply doing things wrong.

Advices from experienced Haskell programmers would be appreciated,

Cheers, Ömer

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

kudah

7:18 p.m.

New subject: Is there an escape from MonadState+MonadIO+MonadError monad stack?

On Sun, 7 Apr 2013 01:02:12 +0300 Ömer Sinan Ağacan wrote:

...

I'm not happy with this design because to me it was like I'm missing the point of using a 'functional' language.

You kind of do, e.g. you might not be able to test parts of your program independently.

...

For instance, in most parts of my code I can actually do IO.

You can disable IO in parts of your code by making it polymorphic over monad. {-# LANGUAGE RankNTypes #-} type MonadStack = ErrorT Err (StateT St IO) type MonadStackNoIO = forall m. Monad m => ErrorT Err (StateT St m)

Tillmann Rendel

6:34 p.m.

New subject: Is there an escape from MonadState+MonadIO+MonadError monad stack?

Hi, Ömer Sinan Ağacan wrote:

...

One thing I'm not happy about my Haskell programs is, almost all of my programs have a monad transformer stack consisting MonadError, MonadIO and MonadState.

You can try to write most of your program in pure functions that are called from a few "main" functions in the monad. Or, if you need some but not all monadic actions in each function, you can use the following pattern: -- This helper function cannot cause monadic effects other than -- throwing errors. But it can be used in arbitrary monads that -- support throwing errors. helper :: MonadError MyError m => ... -> m ... helper = do ... -- Same but with only allowing IO, but other monadic actions other :: MonadIO m => ... -> m ... other = do ... -- we can use both functions in the same monad main = runMyStack $ do helper other This way, you have some control over what effects are allowed where. Tillmann

4429

Age (days ago)

4430

Last active (days ago)

List overview

Download

7 comments

5 participants

participants (5)

Kim-Ee Yeoh
kudah
Roman Cheplyaka
Tillmann Rendel
Ömer Sinan Ağacan