H98 Report: expression syntax glitch

Malcolm Wallace Malcolm.Wallace@cs.york.ac.uk
Mon, 25 Feb 2002 16:30:24 +0000

> Yes, but it reports type errors for the variants
> 	f x = (\x -> x*x .)
> 	g x = (if x then 1 else 2 +)
> and it accepts
> 	h = (let op x y = y in 3 `op`)
> so I suspect it's misparsing these as
> 	f x = (\x -> (x*x .))
> 	g x = (if x then 1 else (2 +))
> 	h = (let op x y = y in (3 `op`))

But I would claim that nhc98 is parsing these correctly, at least
according to section 3 of the Report.  

  " 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. As a consequence, each of these
    constructs has two precedences, one to its left, which is the
    precedence used in the grammar; and one to its right, which is
    obtained via the meta-rule. "

In the table of precedence in the original Report (now deleted in
the revised Report), it makes it clear that a rightward-extending
let, if, or lambda has a lower precedence than an infix operator,
so for instance the parse

  	h = (let op x y = y in (3 `op`))

is correct and

  	h = ((let op x y = y in 3) `op`)

is not.