1 # More detailed differences between Scheme, OCaml, and Haskell #
4 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.
6 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). [Here's another comparison](http://adam.chlipala.net/mlcomp/).
8 In some respects these languages are closer to Scheme than to Haskell: Scheme, OCaml and SML all default to call-by-value evaluation order, and all three have native syntax for mutation and other imperative idioms (though that's not central to their design). Haskell is different in both respects: the default evaluation order is call-by-name (strictly speaking, it's "call-by-need", which is a more efficient cousin), and the only way to have mutation or the like is through the use of monads.
10 On both sides, however, the non-default evaluation order can also be had by using special syntax. And in other respects, OCaml and SML are more like Haskell than they are like Scheme. For example, OCaml and SML and Haskell all permit you to declare types and those types are *statically checked*: that is, your program won't even start to be interpreted unless all the types are consistent. In Scheme, on the other hand, type-checking only happens when your program is running, and the language is generally much laxer about what it accepts as well typed. (There's no problem having a list of mixed numbers and booleans, for example... and you don't need to wrap them in any sum type to do so.)
12 Additionally, the syntax of OCaml and SML is superficially much closer to Haskell's than to Scheme's.
14 # Comments, Whitespace, and Brackets #
16 -- this is a single line comment in Haskell
22 (* this is a single or multiline
25 ; this is a single line comment in Scheme
33 (comment out (a block) (of Scheme code))))
35 * Haskell is sensitive to linespace and indentation: it matters how your code is lined up. OCaml and Scheme don't care about this, though they recommend following some conventions for readability.
37 * 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.
40 We've written some advice on how to do some OCaml-ish and Haskell-ish things in Scheme, and how to get Scheme-ish continuations in OCaml, [[on another page|/rosetta3]].
46 Here we will give some general advice about how to translate between OCaml and Haskell.
50 Here is comparison of the syntax for declaring types in Haskell and OCaml:
53 data Pretty a b = Lovely a | Cute b ClothingModule.ButtonType
54 newtype Pretty a b = Pretty a b Int
55 newtype Pretty a b = Pretty { unPretty a }
56 type Pretty a b = (a, b)
59 type ('a,'b) pretty = Lovely of 'a | Cute of 'b * ClothingModule.ButtonType
60 type ('a,'b) pretty = Pretty of 'a * 'b * int
61 type ('a,'b) pretty = Pretty of 'a
62 type ('a,'b) pretty = 'a * 'b
66 * Our [[more entry-level page|/rosetta1]] comparing Scheme, OCaml, and Haskell (no discussion of types or records)
67 * It may sometimes be useful to try [OCaml](http://try.ocamlpro.com/) or [Haskell](http://tryhaskell.org/) in your web browser
68 * See our pages about [[learning OCaml]] and [[learning Haskell]]
69 * Another page comparing Haskell and OCaml: [Haskell for OCaml Programmers](http://blog.ezyang.com/2010/10/ocaml-for-haskellers/)
70 * Here's the other direction: [Introduction to OCaml for Haskellers](http://foswiki.cs.uu.nl/foswiki/pub/Stc/BeyondFunctionalProgrammingInHaskell:AnIntroductionToOCaml/ocaml.pdf), [another](http://blog.ezyang.com/2010/10/ocaml-for-haskellers/)
71 * Haskell Wiki on [OCaml](https://wiki.haskell.org/OCaml)
72 * [ML Dialects and Haskell](http://hyperpolyglot.org/ml); this discusses other ML-ish languages as well as OCaml and Haskell
73 * Quora discussion of the [differences between Haskell and ML languages](http://www.quora.com/What-are-the-key-differences-between-Haskell-and-Standard-ML?browse)
74 * [Another discussion](http://jxyzabc.blogspot.com/2009/03/haskell-vs-ocaml-or-ravings-of.html)
79 * There are many Haskell tutorials and textbooks available. This is probably the most actively developed: [Haskell wikibook](http://en.wikibooks.org/wiki/Haskell)
80 * [Yet Another Haskell Tutorial](http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf) (much of this excellent book has supposedly been integrated into the Haskell wikibook)
81 * All About Monads has supposedly also been integrated into the Haskell wikibook
82 * (A not-so-)[Gentle Introduction to Haskell](http://web.archive.org/web/http://www.haskell.org/tutorial/) (archived)
83 * [Learn You a Haskell for Great Good](http://learnyouahaskell.com/)
89 * 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`.
91 * 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:
93 data Either a b = Left a | Right b;
94 data FooType a b = Foo_constructor1 a b | Foo_constructor2 a b;
96 In printed media, Haskell type variables are often written using Greek letters, like this:
98 <pre><code>type Either α β = Left α | Right β
101 Some terminology: in this type declaration, `Either` is known as a *type-constructor*, since it takes some types <code>α</code> and <code>β</code> as arguments and yields a new type. We call <code>Left α</code> one of the *variants* for the type <code>Either α β</code>. `Left` and `Right` are known as *value constructors* or *data constructors* or just *constructors*. You can use `Left` in any context where you need a function, for example:
105 In OCaml, value constructors are still capitalized, but type names are lowercase. Type variables take the form `'a` instead of `a`, and if there are multiple type variables, they're not Curried but instead have to be grouped in a tuple. The syntax for whether they appear first or second is also somewhat different. So we have instead:
107 type ('a,'b) either = Left of 'a | Right of 'b;;
108 type ('a,'b) foo_type = Foo_constructor1 of 'a * 'b | Foo_constructor2 of 'a * 'b;;
110 In OCaml, constructors aren't full-fledged functions, so you need to do this instead:
112 List.map (fun x -> Left x) [1; 2]
114 Apart from these differences, there are many similarities between Haskell's and OCaml's use of constructors. For example, in both languages you can do:
116 let Left x = Left 1 in x + 1
118 * In addition to the `data` keyword, Haskell also sometimes uses `type` and `newtype` to declare types. `type` is used just to introduce synonyms. If you say:
120 type Weight = Integer
121 type Person = (Name, Address) -- supposing types Name and Address to be declared elsewhere
123 then you can use a value of type `Integer` wherever a `Weight` is expected, and vice versa. <!-- `type` is allowed to be parameterized -->
125 `newtype` and `data` on the other hand, create genuinely new types. `newtype` is basically just an efficient version of `data` that you can use in special circumstances. `newtype` must always take one type argument and have one value constructor. For example:
127 newtype PersonalData a = PD a
131 data PersonalData2 a = PD2 a
133 And `data` also allows multiple type arguments, and multiple variants and value constructors. <!-- Subtle difference: whereas `PersonalData a` is isomorphic to `a`, `PersonalData2 a` has an additional value, namely `PD2 _|_`. In a strict language, this is an additional type an expression can have, but it would not be a value. -->
135 OCaml just uses the one keyword `type` for all of these purposes:
138 type person = name * address;;
139 type 'a personal_data = PD of 'a;;
141 * When a type only has a single variant, as with PersonalData, Haskell programmers will often use the same name for both the type and the value constructor, like this:
143 data PersonalData3 a = PersonalData3 a
145 The interpreter can always tell from the context when you're using the type name and when you're using the value constructor.
147 * The type constructors discussed above took simple types as arguments. In Haskell, types are also allowed to take *type constructors* as arguments:
149 data BarType t = Bint (t Integer) | Bstring (t string)
151 One does this for example when defining monad transformers---the type constructor `ReaderT` takes some base monad's type constructor as an argument.
153 The way to do this this in OCaml is less straightforward. [See here](/code/tree_monadize.ml) for an example.
155 * Haskell has a notion of *type-classes*. They look like this:
158 (==) :: a -> a -> Bool
160 This declares the type-class `Eq`; in order to belong to this class, a type `a` will have to supply its own implementation of the function `==`, with the type `a -> a -> Bool`. Here is how the `Integer` class signs up to join this type-class:
162 instance Eq Integer where
163 x == y = ... some definition for the Integer-specific version of that function here ...
165 Type expressions can be conditional on some of their parameters belonging to certain type-classes. For example:
167 elem :: (Eq a) => a -> [a] -> Bool
169 says that the function `elem` is only defined over types `a` that belong to the type-class `Eq`. For such types `a`, `elem` has the type `a -> [a] -> Bool`.
173 instance (Eq a) => Eq (Tree a) where
174 Leaf a == Leaf b = a == b
175 (Branch l1 r1) == (Branch l2 r2) = (l1==l2) && (r1==r2)
178 says that if `a` belongs to the typeclass `Eq`, then so too does `Tree a`, and in such cases here is the implementation of `==` for `Tree a`...
180 * OCaml doesn't have type-classes. You can do something similar with OCaml modules that take are parameterized on other modules. Again, [see here](/code/tree_monadize.ml) for an example.
183 * Some specific differences in how certain types are expressed. This block in Haskell:
185 Prelude> type Maybe a = Nothing | Just a
186 Prelude> let x = [] :: [Int]
189 Prelude> let x = () :: ()
190 Prelude> let x = (1, True) :: (Int, Bool)
192 corresponds to this block in OCaml:
194 # type 'a option = None | Some of 'a;;
195 type 'a option = None | Some of 'a
196 # let (x : int list) = [];;
197 val x : int list = []
198 # let (x : unit) = ();;
200 # let (x : int * bool) = (1, true);;
201 val x : int * bool = (1, true)
203 * Haskell has a plethora of numerical types, including the two types `Int` (integers limited to a machine-dependent range) and `Integer` (unbounded integers). The same arithmetic operators (`+` and so on) work for all of these. OCaml also has several different numerical types (though not as many). In OCaml, by default, one has to use a different numerical operator for each type:
208 Error: This expression has type float but an expression was expected of type int
212 However the comparison operators are polymorphic. You can equally say:
223 But you must still apply these operators to expressions of the same type:
226 Error: This expression has type int but an expression was expected of type float
228 * We'll discuss differences between Haskell's and OCaml's record types below.
231 ##Lists, Tuples, Unit, Booleans##
233 * 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.
235 * Haskell describes the boolean type as `Bool` and its variants are `True` and `False`. OCaml describes the type as `bool` and its variants are `true` and `false`. This is an inconsistency in OCaml: other value constructors must always be capitalized.
237 * As noted above, in Haskell one builds up a list by saying `1 : [2, 3]`. In OCaml one says `1 :: [2; 3]`. In Haskell, one can test whether a list is empty with either:
242 In OCaml, there is no predefined `null` or `isempty` function. One can still test whether a list is empty using the comparison `lst = []`.
244 * 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.
246 * Haskell has *list comprehensions*:
248 [ x * x | x <- [1..10], odd x]
250 In OCaml, one has to write this out longhand:
252 List.map (fun x -> x * x) (List.filter odd [1..10]);;
254 * 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.
256 Haskell uses the operator `++` for appending both strings and lists (since Haskell strings are just one kind of list). OCaml uses different operators:
258 # "string1" ^ "string2";;
259 - : string = "string1string2"
260 # ['s';'t'] @ ['r';'i';'n';'g'];;
261 - : char list = ['s'; 't'; 'r'; 'i'; 'n'; 'g']
262 # (* or equivalently *)
263 List.append ['s';'t'] ['r';'i';'n';'g'];;
264 - : char list = ['s'; 't'; 'r'; 'i'; 'n'; 'g']
269 * Haskell permits both:
278 foo x = result1 + result2
279 where result1 = x * x
286 in let result2 = x + 1
287 in result1 + result2;;
293 # let (x, y) as both = (1, 2)
295 - : (int * int) * int * int = ((1, 2), 1, 2)
300 let both@(x,y) = (1, 2)
305 match list_expression with
306 | y::_ when odd y -> result1
307 | y::_ when y > 5 -> result2
308 | y::_ as whole -> (whole, y)
313 case list_expression of
314 (y:_) | odd y -> result1
316 whole@(y:_) -> (whole, y)
322 Haskell and OCaml both have `records`, which are essentially just tuples with a pretty interface. We introduced these in the wiki notes [here](/coroutines_and_aborts/).
324 The syntax for declaring and using these is a little bit different in the two languages.
326 * In Haskell one says:
328 -- declare a record type
329 data Color = Col { red, green, blue :: Int }
330 -- create a value of that type
331 let c = Col { red = 0, green = 127, blue = 255 }
333 In OCaml one says instead:
335 type color = { red : int; green : int; blue : int };;
336 let c = { red = 0; green = 127; blue = 255 }
338 Notice that OCaml doesn't use any value constructor `Col`. The record syntax `{ red = ...; green = ...; blue = ... }` is by itself the constructor. The record labels `red`, `green`, and `blue` cannot be re-used for any other record type.
340 * In Haskell, one may have multiple constructors for a single record type, and one may re-use record labels within that type, so long as the labels go with fields of the same type:
342 data FooType = Constructor1 {f :: Int, g :: Float} | Constructor2 {f :: Int, h :: Bool}
344 * In Haskell, one can extract a single field of a record like this:
346 let c = Col { red = 0, green = 127, blue = 255 }
347 in red c -- evaluates to 0
351 let c = { red = 0; green = 127; blue = 255 }
352 in c.red (* evaluates to 0 *)
354 * In both languages, there is a special syntax for creating a copy of an existing record, with some specified fields altered. In Haskell:
356 let c2 = c { green = 50, blue = 50 }
357 -- evaluates to Col { red = red c, green = 50, blue = 50 }
361 let c2 = { c with green = 50; blue = 50 }
362 (* evaluates to { red = c.red; green = 50; blue = 50 }
364 * One pattern matches on records in similar ways. In Haskell:
366 let Col { red = r, green = g } = c
371 let { red = r; green = g; _ } = c
376 makegray c@(Col { red = r } ) = c { green = r, blue = r }
380 makegray c = let Col { red = r } = c
381 in { red = r, green = r, blue = r }
385 # let makegray ({ red = r; _ } as c) = { c with green=r; blue=r };;
386 val makegray : color -> color = <fun>
387 # makegray { red = 0; green = 127; blue = 255 };;
388 - : color = {red = 0; green = 0; blue = 0}
390 * Records just give your types a pretty interface; they're entirely dispensable. Instead of:
392 type color = { red : int; green : int; blue : int };;
393 let c = { red = 0; green = 127; blue = 255 };;
396 You could instead just use a more familiar data constructor:
398 type color = Color of (int * int * int);;
399 let c = Color (0, 127, 255);;
401 and then extract the field you want using pattern-matching:
403 let Color (r, _, _) = c;;
405 match c with Color (r, _, _) -> ...
407 (Or you could just use bare tuples, without the `Color` data constructor.)
409 The record syntax only exists because programmers sometimes find it more convenient to say:
413 than to reach for those pattern-matching constructions.
419 * 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:
421 factorial :: int -> int
423 factorial n = n * factorial (n-1)
425 In OCaml you must explicitly say when a function is recursive; and this would be written instead as:
427 let rec factorial (n : int) : int =
430 | x -> x * factorial (x-1)
434 let rec factorial : int -> int =
435 fun n -> match n with
437 | x -> x * factorial (x-1)
439 or (though we recommend not using this last form):
441 let rec factorial : int -> int =
444 | x -> x * factorial (x-1)
446 * Another example, in Haskell:
448 length :: [a] -> Integer
450 length (x:xs) = 1 + length xs
454 let rec length : 'a list -> int =
455 fun lst -> match lst with
457 | x::xs -> 1 + length xs
459 * Another example, in Haskell:
467 let sign x = match x with
468 | x' when x' > 0 -> 1
469 | x' when x' = 0 -> 0
472 * In Haskell the equality comparison operator is `==`, and the non-equality operator is `/=`. In OCaml, `==` expresses "physical identity", which has no analogue in Haskell because Haskell has no mutable types. See our discussion of "Four grades of mutation involvement" in the [[Week9]] notes. In OCaml the operator corresponding to Haskell's `==` is just `=`, and the corresponding non-equality operator is `<>`.
474 * In both Haskell and OCaml, one can use many infix operators as prefix functions by parenthesizing them. So for instance:
478 will work in both languages. One notable exception is that in OCaml you can't do this with the list constructor `::`:
482 # (fun x xs -> x :: xs) 1 [1; 2];;
483 - : int list = [1; 1; 2]
485 * Haskell also permits two further shortcuts here that OCaml has no analogue for. In Haskell, in addition to writing:
489 you can also write either of:
494 In OCaml one has to write these out longhand:
499 Also, in Haskell, there's a special syntax for using what are ordinarily prefix functions as infix operators:
501 Prelude> elem 1 [1, 2]
503 Prelude> 1 `elem` [1, 2]
506 In OCaml one can't do that. There's only:
508 # List.mem 1 [1; 2];;
511 * In Haskell one writes anonymous functions like this:
519 * Haskell uses the period `.` as a composition operator:
525 In OCaml one has to write it out longhand:
529 * In Haskell, expressions like this:
537 (Think of the period in our notation for the untyped lambda calculus.)
539 * The names of standard functions, and the order in which they take their arguments, may differ. In Haskell:
542 foldr :: (a -> b -> b) -> b -> [a] -> b
547 - : ('a -> 'b -> 'b) -> 'a list -> 'b -> 'b = <fun>
549 * Some functions are predefined in Haskell but not in OCaml. Here are OCaml definitions for some common ones:
553 let flip f x y = f y x;;
554 let curry (f : ('a, 'b) -> 'c) = fun x y -> f (x, y);;
555 let uncurry (f : 'a -> 'b -> 'c) = fun (x, y) -> f x y;;
556 let null lst = lst = [];;
558 `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.
560 * The `until` function in Haskell is used like this:
562 until (\l -> length l == 4) (1 : ) []
563 -- evaluates to [1,1,1,1]
565 until (\x -> x == 10) succ 0
568 This can be defined in OCaml as:
570 let rec until test f z =
571 if test z then z else until test f (f z)
576 * 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.
578 * Like Scheme and most other languages, OCaml is by default eager. Laziness can be achieved either by using thunks:
580 # let eval_later1 () = 2 / 2;;
581 val eval_later1 : unit -> int = <fun>
582 # let eval_later2 () = 2 / 0;;
583 val eval_later2 : unit -> int = <fun>
587 Exception: Division_by_zero.
589 or by using the special forms `lazy` and `Lazy.force`:
591 # let eval_later3 = lazy (2 / 2);;
592 val eval_later3 : int lazy_t = <lazy>
593 # Lazy.force eval_later3;;
596 - : int lazy_t = lazy 1
598 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`.
603 Haskell has more built-in support for monads, but one can define the monads one needs in OCaml.
605 * 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:
609 In Haskell, one uses the infix operator `>>=` to express bind instead:
613 If you like this Haskell convention, you can define `>>=` in OCaml like this:
617 * Haskell also uses the operator `>>`, where `u >> v` means the same as `u >>= \_ -> v`.
619 * 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.
621 * 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`:
630 This is equivalent in meaning to the following:
638 which can be translated straightforwardly into OCaml.
640 For more details, see:
642 * [Haskell wikibook on do-notation](http://en.wikibooks.org/wiki/Haskell/do_Notation)
643 * [Yet Another Haskell Tutorial on do-notation](http://en.wikibooks.org/wiki/Haskell/YAHT/Monads#Do_Notation)
644 * [Do-notation considered harmful](http://www.haskell.org/haskellwiki/Do_notation_considered_harmful)
646 * If you like the Haskell do-notation, there's [a library](http://www.cas.mcmaster.ca/~carette/pa_monad/) you can compile and install to let you use something similar in OCaml.
648 * In order to do any printing, Haskell has to use a special `IO` monad. So programs will look like this:
652 let s = "hello world"
657 let s = "hello world"
662 main = let s = "hello world"
663 in putStrLn s >> return s
665 OCaml permits you to mix side-effects with regular code, so you can just print, without needing to bring in any monad:
668 let s = "hello world"
669 in let () = print_endline s
675 let s = "hello world"
676 in print_endline s ; s;;