Haskell or Standard ML for beginners? [closed]

Much as I love Haskell, here are the reasons I would prefer SML for a class in discrete math and data structures (and most other beginners’ classes):

• Time and space costs of Haskell programs can be very hard to predict, even for experts. SML offers much more limited ways to blow the machine.

• Syntax for function defintion in an interactive interpreter is identical to syntax used in a file, so you can cut and paste.

• Although operator overloading in SML is totally bogus, it is also simple. It’s going to be hard to teach a whole class in Haskell without having to get into type classes.

• Student can debug using print. (Although, as a commenter points out, it is possible to get almost the same effect in Haskell using Debug.Trace.trace.)

• Infinite data structures blow people’s minds. For beginners, you’re better off having them define a stream type complete with ref cells and thunks, so they know how it works:

  datatype 'a thunk_contents = UNEVALUATED of unit -> 'a
                             | VALUE of 'a
  type 'a thunk = 'a thunk_contents ref
  val delay : (unit -> 'a) -> 'a thunk
  val force : 'a thunk -> 'a
  

  Now it’s not magic any more, and you can go from here to streams (infinite lists).

• Layout is not as simple as in Python and can be confusing.

There are two places Haskell has an edge:

• In core Haskell you can write a function’s type signature just before its definition. This is hugely helpful for students and other beginners. There just isn’t a nice way to deal with type signatures in SML.

• Haskell has better concrete syntax. The Haskell syntax is a major improvement over ML syntax. I have written a short note about when to use parentheses in an ML program; this helps a little.

Finally, there is a sword that cuts both ways:

• Haskell code is pure by default, so your students are unlikely to stumble over impure constructs (IO monad, state monad) by accident. But by the same token, they can’t print, and if you want to do I/O then at minumum you have to explain do notation, and return is confusing.

On a related topic, here is some advice for your course preparation: don’t overlook Purely Functional Data Structures by Chris Okasaki. Even if you don’t have your students use it, you will definitely want to have a copy.