unexpected token "where" when indentation of do-block same as where

[mabre]

The following code, which compiles with ghc, only compiles with frege when removing a space before where

module DoWhere where

main = do
    print x
    where x = 1

E DoWhere.hs:5: unexpected token "where" while trying to
    parse do expression qualifiers
E DoWhere.hs:3: last statement in a monadic do block must
    not be let decls
E DoWhere.hs:5: syntax error on '}'

[Ingo60]

This seems to employ the following provision in the Haskell report:

A close brace is also inserted whenever the syntactic category containing the layout list ends; that is, if an illegal lexeme is encountered at a point where a close brace would be legal, a close brace is inserted.

This is impossible in Frege, as the layout is done in the lexical analyser.

[Dierk]

How about dealing with this special case by just adding a closing brace before any "where"?

[Ingo60]

Like in

module Main } where

Dierk???

[Dierk]

well, just a bit more intelligently ;-)
Whereas it may be that the "intelligent" bit is the difficult part...

[Ingo60]

We can, at a later date, explore additional possibilities.
Theoretically, we could have several rules of the form: Instead of issuing a syntax error, insert X in a place where X would be allowed.

Commas, Braces, Parentheses, Brackets, Semicolons, keywords, whatever you want.
It's only making everything more complicated, including resolution of genuine syntax errors. ("Why does it complain about }}}foo when I don't have braces in that line?")

[Dierk]

true.

From a pure user standpoint, I would certainly like to have "where" inside "do" blocks - and at the moment I would trade the quality of error messages for that feature. (just my two cents)

[Ingo60]

A where block can follow any case alternative or RHS of a function or pattern binding. You can, of course, have case expressions and bindings inside a do .