"if ... then \case -> else ..." causes a "missing else clause" error

The following program:

{-# LANGUAGE LambdaCase #-}

foo =
    if True
    then
        \case ->
            1 -> 2
    else id

Produces this message:

problem.hs:4:5:
    parse error in if statement: missing required else clause

The problem in the code is that there's a -> after the \case.

I don't know if a better error message can be generated, but this one is really unhelpful.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import parser labels

I ran into this in another more complex program yesterday. It took me 15 mins to find out the real cause of the error.

Regression since 7.8 ("parse error on input ‘->’"), so I guess it is caused by D201.

changed milestone to %7.10.3

@MikeIzbicki: as the author of D201, could you have a look?

I'm sorry, but I doubt I'll have time to look into this anytime soon.

added newcomer label

I'm inclined to just the delete the parser changes from D201. They were supposed to make error messages better, but instead they make them worse.

Here's an example from the testsuite (parser/should_fail/readFail020):

f = let x = 42 } in x
-- before:
parse error on input ‘}’
-- after:
parse error in let binding: missing required 'in'

Here's another:

f = if module then True else False
-- before:
parse error on input ‘module’
-- after:
parse error in if statement: naked if statement

For that last one, the parser code looks like this:

| 'if' error      {% hintIf (getLoc $1) "naked if statement" }
| 'if' exp optSemi error                              {% ... something ... }
| 'if' exp optSemi 'then' error                       {% ... something ... }
| 'if' exp optSemi 'then' exp optSemi error           {% ... something ... }
| 'if' exp optSemi 'then' exp optSemi 'else' error    {% ... something ... }

What this means is that a lexical error in the condition of an if-expression, that we don't handle gracefully in the parser, will be reported as "naked if statement".

The example from the description has the same cause.

Trac metadata

Trac field	Value
Differential revisions	- → D1309

Worth double-checking to see whether the set of shift/reduce conflicts in Parser.y needs updating.

mentioned in commit e2b579e8

added 1 deleted label

mentioned in issue #10988 (closed)

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added Pnormal label

mentioned in merge request !4711 (closed)