Mutability / side effects

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I’ll answer a few of your questions, then provide some links.

Answers

  • How do i understand the ‘source’ of the mutability in Haskell?

    Mutability in Haskell uses the same mechanism as C, usually because:

    • the run-time system for Haskell, and

    • the primitive entities Haskell is based on (e.g. the Integer type and its arithmetic operators primPlusInteger, primMinusInteger, etc)

    are written in an imperative language like C.

  • Is everything written in Haskell?

    No. That would be impractical - that’s why Haskell has an FFI.

  • How do i know if something is mutable?

    Sometimes by looking at the name of the type e.g. IORef, STRef, MVar, STMVar. But there’s no way in general to know if something in Haskell is mutable.

  • What does the Haskell runtime do when it runs the program?

    The Haskell run-time system is “dual-purpose”:

    • it can call regular Haskell functions like (\ x y -> x && not y);

    • and run IO actions like putStrLn "Hello, World!" .

    You can see this in the type signature for IO’s bind operator:

    (>>=) :: IO a         -- run this action to obtain a result,
      -> (a -> IO b)  -- then call this function, using the result as the argument,
      -> IO b         -- to build another action.

    What the run-time system selects then depends on what it’s presented with moment-to-moment as the Haskell program runs.

  • […] I am naturally left to wonder how [Haskell] actually interacts with the outside world.

    Without going into all the possible ways IO can be implemented: very carefully! There are good reasons why the Haskell 2010 report (page 95 of 329) deems IO to be abstract, so until that requirement changes (e.g. in a future version of Haskell) I recommend either:

    • ignoring how IO is implemented altogether - so far, you’ve managed to get by without wondering how (->) is implemented;

    • or looking at several implementations of IO e.g. in How to Declare an Imperative to better understand the advantages and difficulties of each implementation method.

Links

From the wiki:

  1. Introducing I/O

  2. Mutable variable - HaskellWiki

  3. IO inside - HaskellWiki

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