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I'm approaching an intermediate level in Haskell, but I don't really see how writing a program in Haskell is any more like writing a language than writing a program in, say, python. Could you elaborate or point to some DSLs written on top of Haskell? I'm very curious.


Here is a fantastic article to get you started down this path. Note that the author, in his introduction, submits that the approach to FizzBuzz in this paper is a "somewhat tongue-in-cheek solution." Nevertheless, by applying this kind of thinking to what is, in some ways, a deceptively simple-looking problem, the paper serves as a great starting point for using DSLs as an approach to problem-solving.

https://themonadreader.files.wordpress.com/2014/04/fizzbuzz....


There is another article about FizzBuzz I wrote roughly 5 years ago (forgive the pronouns, I need to rebuild the site and I'm procrastinating on the css).

http://dave.fayr.am/posts/2012-10-4-finding-fizzbuzz.html

This uses a slightly different set of abstractions with the same problem.

But perhaps more specifically, when you select a monad stack (group of effects) to compose to solve a problem you're building the features of the language and its effects. And if you use the "final tagless" or "Free" approaches you're doing that even more directly.


You can think a monad defines a DSL. Let's take for example the Rand[0] monad:

    type Rand g = RandT g Identity

    newtype RandT g m a = ...

    -- with
    runRandT :: (Monad m, RandomGen g) => RandT g m a -> g -> m (a, g)
    newtype MyType = Rand StdGen

    -- now you can define
    intGreaterThan :: Integer -> MyType Integer
    intGreaterThan = ...

    randomName :: Integer -> MyType String
    randomName length = ...

    -- an finally convine the two above

    nRandomNames :: Integer -> Integer -> MyType [String]
    nRandomNames length n
       | n <=0 = return []
       | otherwise = do
           tailNames <- nRandomNames (n-1)
           headName <- randomName length
           return (headName:tailNames)
These functions of type `...->MyType a` can be viewed as a DSL where the Random generator state is abstracted away.

[0]hackage.haskell.org/package/MonadRandom-0.3/docs/Control-Monad-Random.html



(From article comments)

    #include <stdio.h>
    
    int
    main(int argc, char **argv)
    {
      double i, s;
      s = 0;
      for (i = 1; i < 100000000; i++)
        s += 1/i;
      printf("Almost infinity is %g\n", s);
    }

    Lennarts-Computer% gcc -O3 inf.c -o inf
    Lennarts-Computer% time ./inf
    Almost infinity is 18.9979
    1.585u 0.009s 0:01.62 97.5%     0+0k 0+0io 0pf+0w
And now the Haskell code:

    import BASIC
    main = runBASIC' $ do
        10 LET I =: 1
        20 LET S =: 0
        30 LET S =: S + 1/I
        40 LET I =: I + 1
        50 IF I <> 100000000 THEN 30
        60 PRINT "Almost infinity is"
        70 PRINT S
        80 END
And running it:

    Lennarts-Computer% ghc --make Main.hs
    [4 of 4] Compiling Main             ( Main.hs, Main.o )
    Linking Main ...
    Lennarts-Computer% ./Main
    Almost infinity is
    18.9979
    CPU time:   1.57s
As you can see it's about the same time. In fact the assembly code for the loops look pretty much the same. Here's the Haskell one:

    LBB1_1: ## _L4
            movsd   LCPI1_0, %xmm2
            movapd  %xmm1, %xmm3
            addsd   %xmm2, %xmm3
            ucomisd LCPI1_1, %xmm3
            divsd   %xmm1, %xmm2
            addsd   %xmm2, %xmm0
            movapd  %xmm3, %xmm1
            jne     LBB1_1  ## _L4
---

OMG that is astonishing


Everyday we stray further from god's light




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