6.2.18. More liberal syntax for function arguments

BlockArguments
Since

8.6.1

Allow do expressions, lambda expressions, etc. to be directly used as a function argument.

In Haskell 2010, certain kinds of expressions can be used without parentheses as an argument to an operator, but not as an argument to a function. They include do, lambda, if, case, and let expressions. Some GHC extensions also define language constructs of this type: mdo (The recursive do-notation), \case (Lambda-case), and proc (Arrow notation).

The BlockArguments extension allows these constructs to be directly used as a function argument. For example:

when (x > 0) do
  print x
  exitFailure

will be parsed as:

when (x > 0) (do
  print x
  exitFailure)

and

withForeignPtr fptr \ptr -> c_memcpy buf ptr size

will be parsed as:

withForeignPtr fptr (\ptr -> c_memcpy buf ptr size)

6.2.18.1. Changes to the grammar

The Haskell report defines the lexp nonterminal thus (* indicates a rule of interest)

lexp  →  \ apat1 … apatn -> exp            (lambda abstraction, n ≥ 1)  *
      |  let decls in exp                  (let expression)             *
      |  if exp [;] then exp [;] else exp  (conditional)                *
      |  case exp of { alts }              (case expression)            *
      |  do { stmts }                      (do expression)              *
      |  fexp

fexp  →  [fexp] aexp                       (function application)

aexp  →  qvar                              (variable)
      |  gcon                              (general constructor)
      |  literal
      |  ( exp )                           (parenthesized expression)
      |  qcon { fbind1 … fbindn }          (labeled construction)
      |  aexp { fbind1 … fbindn }          (labelled update)
      |  …

The BlockArguments extension moves these production rules under aexp

lexp  →  fexp

fexp  →  [fexp] aexp                       (function application)

aexp  →  qvar                              (variable)
      |  gcon                              (general constructor)
      |  literal
      |  ( exp )                           (parenthesized expression)
      |  qcon { fbind1 … fbindn }          (labeled construction)
      |  aexp { fbind1 … fbindn }          (labelled update)
      |  \ apat1 … apatn -> exp            (lambda abstraction, n ≥ 1)  *
      |  let decls in exp                  (let expression)             *
      |  if exp [;] then exp [;] else exp  (conditional)                *
      |  case exp of { alts }              (case expression)            *
      |  do { stmts }                      (do expression)              *
      |  …

Now the lexp nonterminal is redundant and can be dropped from the grammar.

Note that this change relies on an existing meta-rule to resolve ambiguities:

The grammar is ambiguous regarding the extent of lambda abstractions, let expressions, and conditionals. The ambiguity is resolved by the meta-rule that each of these constructs extends as far to the right as possible.

For example, f \a -> a b will be parsed as f (\a -> a b), not as f (\a -> a) b.