8.3. Syntactic extensions

8.3.1. Hierarchical Modules

GHC supports a small extension to the syntax of module names: a module name is allowed to contain a dot ‘.’. This is also known as the “hierarchical module namespace” extension, because it extends the normally flat Haskell module namespace into a more flexible hierarchy of modules.

This extension has very little impact on the language itself; modules names are always fully qualified, so you can just think of the fully qualified module name as “the module name”. In particular, this means that the full module name must be given after the module keyword at the beginning of the module; for example, the module A.B.C must begin

module A.B.C

It is a common strategy to use the as keyword to save some typing when using qualified names with hierarchical modules. For example:

import qualified Control.Monad.ST.Strict as ST

For details on how GHC searches for source and interface files in the presence of hierarchical modules, see Section 5.6.3, “The search path”.

GHC comes with a large collection of libraries arranged hierarchically; see the accompanying library documentation. More libraries to install are available from HackageDB.

8.3.2. Pattern guards

The discussion that follows is an abbreviated version of Simon Peyton Jones's original proposal. (Note that the proposal was written before pattern guards were implemented, so refers to them as unimplemented.)

Suppose we have an abstract data type of finite maps, with a lookup operation:

lookup :: FiniteMap -> Int -> Maybe Int

The lookup returns Nothing if the supplied key is not in the domain of the mapping, and (Just v) otherwise, where v is the value that the key maps to. Now consider the following definition:

clunky env var1 var2 | ok1 && ok2 = val1 + val2
| otherwise  = var1 + var2
  m1 = lookup env var1
  m2 = lookup env var2
  ok1 = maybeToBool m1
  ok2 = maybeToBool m2
  val1 = expectJust m1
  val2 = expectJust m2

The auxiliary functions are

maybeToBool :: Maybe a -> Bool
maybeToBool (Just x) = True
maybeToBool Nothing  = False

expectJust :: Maybe a -> a
expectJust (Just x) = x
expectJust Nothing  = error "Unexpected Nothing"

What is clunky doing? The guard ok1 && ok2 checks that both lookups succeed, using maybeToBool to convert the Maybe types to booleans. The (lazily evaluated) expectJust calls extract the values from the results of the lookups, and binds the returned values to val1 and val2 respectively. If either lookup fails, then clunky takes the otherwise case and returns the sum of its arguments.

This is certainly legal Haskell, but it is a tremendously verbose and un-obvious way to achieve the desired effect. Arguably, a more direct way to write clunky would be to use case expressions:

clunky env var1 var2 = case lookup env var1 of
  Nothing -> fail
  Just val1 -> case lookup env var2 of
    Nothing -> fail
    Just val2 -> val1 + val2
  fail = var1 + var2

This is a bit shorter, but hardly better. Of course, we can rewrite any set of pattern-matching, guarded equations as case expressions; that is precisely what the compiler does when compiling equations! The reason that Haskell provides guarded equations is because they allow us to write down the cases we want to consider, one at a time, independently of each other. This structure is hidden in the case version. Two of the right-hand sides are really the same (fail), and the whole expression tends to become more and more indented.

Here is how I would write clunky:

clunky env var1 var2
  | Just val1 <- lookup env var1
  , Just val2 <- lookup env var2
  = val1 + val2
...other equations for clunky...

The semantics should be clear enough. The qualifiers are matched in order. For a <- qualifier, which I call a pattern guard, the right hand side is evaluated and matched against the pattern on the left. If the match fails then the whole guard fails and the next equation is tried. If it succeeds, then the appropriate binding takes place, and the next qualifier is matched, in the augmented environment. Unlike list comprehensions, however, the type of the expression to the right of the <- is the same as the type of the pattern to its left. The bindings introduced by pattern guards scope over all the remaining guard qualifiers, and over the right hand side of the equation.

Just as with list comprehensions, boolean expressions can be freely mixed with among the pattern guards. For example:

f x | [y] <- x
    , y > 3
    , Just z <- h y
    = ...

Haskell's current guards therefore emerge as a special case, in which the qualifier list has just one element, a boolean expression.

8.3.3. The recursive do-notation

The recursive do-notation (also known as mdo-notation) is implemented as described in A recursive do for Haskell, by Levent Erkok, John Launchbury, Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania. This paper is essential reading for anyone making non-trivial use of mdo-notation, and we do not repeat it here.

The do-notation of Haskell does not allow recursive bindings, that is, the variables bound in a do-expression are visible only in the textually following code block. Compare this to a let-expression, where bound variables are visible in the entire binding group. It turns out that several applications can benefit from recursive bindings in the do-notation, and this extension provides the necessary syntactic support.

Here is a simple (yet contrived) example:

import Control.Monad.Fix

justOnes = mdo xs <- Just (1:xs)
               return xs

As you can guess justOnes will evaluate to Just [1,1,1,....

The Control.Monad.Fix library introduces the MonadFix class. It's definition is:

class Monad m => MonadFix m where
   mfix :: (a -> m a) -> m a

The function mfix dictates how the required recursion operation should be performed. For example, justOnes desugars as follows:

justOnes = mfix (\xs' -> do { xs <- Just (1:xs'); return xs }

For full details of the way in which mdo is typechecked and desugared, see the paper A recursive do for Haskell. In particular, GHC implements the segmentation technique described in Section 3.2 of the paper.

If recursive bindings are required for a monad, then that monad must be declared an instance of the MonadFix class. The following instances of MonadFix are automatically provided: List, Maybe, IO. Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class for Haskell's internal state monad (strict and lazy, respectively).

Here are some important points in using the recursive-do notation:

  • The recursive version of the do-notation uses the keyword mdo (rather than do).

  • It is enabled with the flag -XRecursiveDo, which is in turn implied by -fglasgow-exts.

  • Unlike ordinary do-notation, but like let and where bindings, name shadowing is not allowed; that is, all the names bound in a single mdo must be distinct (Section 3.3 of the paper).

  • Variables bound by a let statement in an mdo are monomorphic in the mdo (Section 3.1 of the paper). However GHC breaks the mdo into segments to enhance polymorphism, and improve termination (Section 3.2 of the paper).

The web page: http://www.cse.ogi.edu/PacSoft/projects/rmb/ contains up to date information on recursive monadic bindings.

Historical note: The old implementation of the mdo-notation (and most of the existing documents) used the name MonadRec for the class and the corresponding library. This name is not supported by GHC.

8.3.4. Parallel List Comprehensions

Parallel list comprehensions are a natural extension to list comprehensions. List comprehensions can be thought of as a nice syntax for writing maps and filters. Parallel comprehensions extend this to include the zipWith family.

A parallel list comprehension has multiple independent branches of qualifier lists, each separated by a `|' symbol. For example, the following zips together two lists:

   [ (x, y) | x <- xs | y <- ys ] 

The behavior of parallel list comprehensions follows that of zip, in that the resulting list will have the same length as the shortest branch.

We can define parallel list comprehensions by translation to regular comprehensions. Here's the basic idea:

Given a parallel comprehension of the form:

   [ e | p1 <- e11, p2 <- e12, ... 
       | q1 <- e21, q2 <- e22, ... 

This will be translated to:

   [ e | ((p1,p2), (q1,q2), ...) <- zipN [(p1,p2) | p1 <- e11, p2 <- e12, ...] 
                                         [(q1,q2) | q1 <- e21, q2 <- e22, ...] 

where `zipN' is the appropriate zip for the given number of branches.

8.3.5. Rebindable syntax

GHC allows most kinds of built-in syntax to be rebound by the user, to facilitate replacing the Prelude with a home-grown version, for example.

You may want to define your own numeric class hierarchy. It completely defeats that purpose if the literal "1" means "Prelude.fromInteger 1", which is what the Haskell Report specifies. So the -XNoImplicitPrelude flag causes the following pieces of built-in syntax to refer to whatever is in scope, not the Prelude versions:

  • An integer literal 368 means "fromInteger (368::Integer)", rather than "Prelude.fromInteger (368::Integer)".

  • Fractional literals are handed in just the same way, except that the translation is fromRational (3.68::Rational).

  • The equality test in an overloaded numeric pattern uses whatever (==) is in scope.

  • The subtraction operation, and the greater-than-or-equal test, in n+k patterns use whatever (-) and (>=) are in scope.

  • Negation (e.g. "- (f x)") means "negate (f x)", both in numeric patterns, and expressions.

  • "Do" notation is translated using whatever functions (>>=), (>>), and fail, are in scope (not the Prelude versions). List comprehensions, mdo (Section 8.3.3, “The recursive do-notation ”), and parallel array comprehensions, are unaffected.

  • Arrow notation (see Section 8.9, “Arrow notation ”) uses whatever arr, (>>>), first, app, (|||) and loop functions are in scope. But unlike the other constructs, the types of these functions must match the Prelude types very closely. Details are in flux; if you want to use this, ask!

In all cases (apart from arrow notation), the static semantics should be that of the desugared form, even if that is a little unexpected. For example, the static semantics of the literal 368 is exactly that of fromInteger (368::Integer); it's fine for fromInteger to have any of the types:

fromInteger :: Integer -> Integer
fromInteger :: forall a. Foo a => Integer -> a
fromInteger :: Num a => a -> Integer
fromInteger :: Integer -> Bool -> Bool

Be warned: this is an experimental facility, with fewer checks than usual. Use -dcore-lint to typecheck the desugared program. If Core Lint is happy you should be all right.

8.3.6. Postfix operators

GHC allows a small extension to the syntax of left operator sections, which allows you to define postfix operators. The extension is this: the left section

  (e !)

is equivalent (from the point of view of both type checking and execution) to the expression

  ((!) e)

(for any expression e and operator (!). The strict Haskell 98 interpretation is that the section is equivalent to

  (\y -> (!) e y)

That is, the operator must be a function of two arguments. GHC allows it to take only one argument, and that in turn allows you to write the function postfix.

Since this extension goes beyond Haskell 98, it should really be enabled by a flag; but in fact it is enabled all the time. (No Haskell 98 programs change their behaviour, of course.)

The extension does not extend to the left-hand side of function definitions; you must define such a function in prefix form.

8.3.7. Record field disambiguation

In record construction and record pattern matching it is entirely unambiguous which field is referred to, even if there are two different data types in scope with a common field name. For example:

module M where
  data S = MkS { x :: Int, y :: Bool }

module Foo where
  import M

  data T = MkT { x :: Int }
  ok1 (MkS { x = n }) = n+1   -- Unambiguous

  ok2 n = MkT { x = n+1 }     -- Unambiguous

  bad1 k = k { x = 3 }  -- Ambiguous
  bad2 k = x k          -- Ambiguous

Even though there are two x's in scope, it is clear that the x in the pattern in the definition of ok1 can only mean the field x from type S. Similarly for the function ok2. However, in the record update in bad1 and the record selection in bad2 it is not clear which of the two types is intended.

Haskell 98 regards all four as ambiguous, but with the -fdisambiguate-record-fields flag, GHC will accept the former two. The rules are precisely the same as those for instance declarations in Haskell 98, where the method names on the left-hand side of the method bindings in an instance declaration refer unambiguously to the method of that class (provided they are in scope at all), even if there are other variables in scope with the same name. This reduces the clutter of qualified names when you import two records from different modules that use the same field name.