Martin Erwig wrote:
Hi there,
I am trying to use the "let"-notation within do-expressions, and it seems to me that it does not work correctly. (I am using Hugs, version February 2000, on Solaris.)
E.g.,
do {x <- [1..9]; let i=1 in do {return ((+i) x)}}
works as expected, but with
do {x <- [1..9]; let i=1; return ((+i) x)}
I get:
ERROR: Syntax error in definition (unexpected symbol "i")
According to the Report (Section 3.14) the latter expression should be a valid abbreviation of the former, or am I missing something here? If it is a bug, has it been corrected in the new version of Hugs?
No, it's not a bug, but an unfortunate consequence of the current definition of the layout rule. What you've written will be parsed as do {x <- [1..9]; let {i=1; return ((+i) x)}} which is clearly incorrect (although the error message isn't very helpful). What you probably intended was do {x <- [1..9]; let {i=1}; return ((+i) x)} but the layout rule isn't advanced enough to figure this out. The exact reason is that only one look-ahead is used to determine if a closing brace should be inserted, but since 'return' is a valid prefix of a let-definition, the parser has already committed to parsing more let-bindings when the error is detected. Note especially that do {x <- [1..9]; let i=1; in return ((+i) x)} parses ok. The problem with this part of the layout rule was discussed on the haskell list not long ago, see http://www.mail-archive.com/haskell@haskell.org/msg08099.html -- Johan