+[[!toc]]
+
The functional programming literature tends to use one of four languages: Scheme, OCaml, Standard ML (SML), or Haskell. With experience, you'll grow comfortable switching between these. At the beginning, though, it can be confusing.
The easiest translations are between OCaml and SML. These languages are both derived from a common ancestor, ML. For the most part, the differences between them are only superficial. [Here's a translation manual](http://www.mpi-sws.org/~rossberg/sml-vs-ocaml.html).
Additionally, the syntax of OCaml and SML is superficially much closer to Haskell's than to Scheme's.
-##Comments, Whitespace, and Brackets##
+#Comments, Whitespace, and Brackets#
-- this is a single line comment in Haskell
* In Haskell, a block of code can be bracketed with `{` and `}`, with different expressions separated by `;`. But usually one would use line-breaks and proper indentation instead. In OCaml, separating expressions with `;` has a different meaning, having to do with how side-effects are sequenced. Instead, one can bracket a block of code with `(` and `)` or with `begin` and `end`. In Scheme, of course, every parentheses is significant.
-##Scheme and OCaml##
+#Scheme and OCaml#
* You can [try Scheme in your web browser](http://tryscheme.sourceforge.net/). This is useful if you don't have Racket or another Scheme implementation installed---but don't expect it to have all the bells and whistles of a mature implementation!
We won't say any more about translating to and from Scheme.
-##Haskell and OCaml##
+#Haskell and OCaml#
We will however try to give some general advice about how to translate between OCaml and Haskell.
* [Learn You a Haskell for Great Good](http://learnyouahaskell.com/)
-#Type expressions#
+##Type expressions##
* In Haskell, you say a value has a certain type with: `value :: type`. You express the operation of prepending a new `int` to a list of `int`s with `1 : other_numbers`. In OCaml it's the reverse: you say `value : type` and `1 :: other_numbers`.
-* In Haskell, type names and constructors both begin with capital letters, and type variables always appear after their constructors, in Curried form. And the primary term for declaring a new type is `data` (short for "abstract datatype").
-So we have:
+* In Haskell, type names and constructors both begin with capital letters, and type variables always appear after their constructors, in Curried form. And the primary term for declaring a new type is `data` (short for [[!wikipedia algebraic data type]]). So we have:
data Either a b = Left a | Right b;
data FooType a b = Foo_constructor1 a b | Foo_constructor2 a b;
* We'll discuss differences between Haskell's and OCaml's record types below.
-#Lists, Tuples, Unit, Booleans#
+##Lists, Tuples, Unit, Booleans##
* As noted above, Haskell describes the type of a list of `Int`s as `[Int]`. OCaml describes it as `int list`. Haskell describes the type of a pair of `Int`s as `(Int, Int)`. OCaml describes it as `int * int`. Finally, Haskell uses `()` to express both the unit type and a value of that type. In OCaml, one uses `()` for the value and `unit` for the type.
In OCaml, there is no predefined `null` or `isempty` function. One can still test whether a list is empty using the comparison `lst = []`.
-* In Haskell, the expression [1..5] is the same as [1,2,3,4,5], and the expression [0..] is a infinite lazily-evaluated stream of the natural numbers. In OCaml, there is no [1..5] shortcut, lists must be finite, and they are eagerly evaluated. It is possible to create lazy streams in OCaml, even infinite ones, but you have to use other techniques than the native list type.
+* In Haskell, the expression `[1..5]` is the same as `[1,2,3,4,5]`, and the expression `[0..]` is a infinite lazily-evaluated stream of the natural numbers. In OCaml, there is no `[1..5]` shortcut, lists must be finite, and they are eagerly evaluated. It is possible to create lazy streams in OCaml, even infinite ones, but you have to use other techniques than the native list type.
* Haskell has *list comprehensions*:
List.map (fun x -> x * x) (List.filter odd [1..10]);;
-* In Haskell, the expressions "abc" and ['a','b','c'] are equivalent. (Strings are just lists of chars.) In OCaml, these expressions have two different types.
+* In Haskell, the expressions `"abc"` and `['a','b','c']` are equivalent. (Strings are just lists of `char`s.) In OCaml, these expressions have two different types.
Haskell uses the operator `++` for appending both strings and lists (since Haskell strings are just one kind of list). OCaml uses different operators:
- : char list = ['s'; 't'; 'r'; 'i'; 'n'; 'g']
-#Let and Where#
+##Let and Where##
* Haskell permits both:
in let result2 = x + 1
in result1 + result2;;
-#Patterns#
+##Patterns##
* In OCaml:
[] -> result4
-#Records#
+##Records##
Haskell and OCaml both have `records`, which are essentially just tuples with a pretty interface. The syntax for declaring and using these is a little bit different in the two languages.
- : color = {red = 0; green = 0; blue = 0}
-#Functions#
+##Functions##
* In Haskell functions are assumed to be recursive, and their types and applications to values matching different patterns are each declared on different lines. So we have:
let uncurry (f : 'a -> 'b -> 'c) = fun (x, y) -> f x y;;
let null lst = lst = [];;
- `fst` and `snd` (defined only on pairs) are provided in both languages. Haskell has `head` and `tail` for lists; these will raise an exception if applied to []. In OCaml the corresponding functions are `List.hd` and `List.tl`. Many other Haskell list functions like `length` are available in OCaml as `List.length`, but OCaml's standard libraries are leaner that Haskell's.
+ `fst` and `snd` (defined only on pairs) are provided in both languages. Haskell has `head` and `tail` for lists; these will raise an exception if applied to `[]`. In OCaml the corresponding functions are `List.hd` and `List.tl`. Many other Haskell list functions like `length` are available in OCaml as `List.length`, but OCaml's standard libraries are leaner that Haskell's.
* The `until` function in Haskell is used like this:
This can be defined in OCaml as:
- let rec until test f z =
- if test z then z else until test f (f z)
+ let rec until test f z =
+ if test z then z else until test f (f z)
-#Lazy or Eager#
+##Lazy or Eager##
* As we've mentioned several times, Haskell's evaluation is by default *lazy* or "call-by-need" (that's an efficient version of "call-by-name" that avoids computing the same results again and again). In some places Haskell will force evaluation to be *eager* or "strict". This is done in several different ways; the symbols `!` and `seq` are signs that it's being used.
# eval_later3;;
- : int lazy_t = lazy 1
- Notice in the last line the value is reported as being `lazy 1` instead of `<lazy>`. Since the value has once been forced, it won't ever need to be recomputed. The thunks are less efficient in this respect. Even though OCaml will now remember that `eval_later3` should be forced to, `eval_later3` is still type distinct from a plain `int`.
+ Notice in the last line the value is reported as being `lazy 1` instead of `<lazy>`. Since the value has once been forced, it won't ever need to be recomputed. The thunks are less efficient in this respect. Even though OCaml will now remember what `eval_later3` should be forced to, `eval_later3` is still type-distinct from a plain `int`.
-#Monads#
+##Monads##
Haskell has more built-in support for monads, but one can define the monads one needs in OCaml.
-* In our seminar, we've been calling one monadic operation `unit`, in Haskell the same operation is called `return`. We've been calling another monadic operation `bind`, used in prefix form, like this:
+* In our seminar, we've been calling one monadic operation `unit`; in Haskell the same operation is called `return`. We've been calling another monadic operation `bind`, used in prefix form, like this:
bind u f
u >>= f
- If you like this Haskell convention, you can define (>>=) in OCaml like this:
+ If you like this Haskell convention, you can define `>>=` in OCaml like this:
let (>>=) = bind;;
* Haskell also uses the operator `>>`, where `u >> v` means the same as `u >>= \_ -> v`.
-* In Haskell, one can generally just use plain `return` and `>>=` and the compiler will infer what monad you must be talking about from the surrounding type constraints. In OCaml, you generally need to be specific about which monad you're using. So in these notes, when mutiple monads are on the table, we've defined operations as `reader_unit` and `reader_bind`.
+* In Haskell, one can generally just use plain `return` and `>>=` and the interpreter will infer what monad you must be talking about from the surrounding type constraints. In OCaml, you generally need to be specific about which monad you're using. So in these notes, when mutiple monads are on the table, we've defined operations as `reader_unit` and `reader_bind`, and so on.
* Haskell has a special syntax for working conveniently with monads. It looks like this. Assume `u` `v` and `w` are values of some monadic type `M a`. Then `x` `y` and `z` will be variables of type `a`:
v >>= \ y ->
w >>= \ _ ->
let z = foo x y
- in unit z
+ in return z
which can be translated straightforwardly into OCaml.