Apparently the only possible interpretation of runSomeMonad do ... is runSomeMonad (do ...). Why isn't the first variant allowed by the Haskell syntax? Is there some case where foo do bar could be actually ambiguous?
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7Not just do, `foo if bar then quux else baz` as well. – daniel gratzer Feb 20 '14 at 19:27
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2It probably has something to do with the fact that you can have something like `runSomeMonad (do ...) x y z ...`. – David Young Feb 20 '14 at 20:44
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2@DavidYoung I don't see how the proposed grammar change would prevent that kind of thing. – Daniel Wagner Feb 20 '14 at 22:51
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2Just a historical accident, as far as I know. – augustss Feb 20 '14 at 23:38
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1As of GHC 8.6.1, you can enable the [`BlockArguments` extension](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#extension-BlockArguments) to allow, among other things, `runSomeMonad do ...`. – chepner Nov 13 '18 at 17:51
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Note that you can observe this effect with not just do, but also let, if, \, case, the extensions mdo and proc…and the dread unary -. I cannot think of a case in which this is ambiguous except for unary -. Here’s how the grammar is defined in the Haskell 2010 Language Report, §3: Expressions.
exp
→ infixexp :: [context =>] type
| infixexp
infixexp
→ lexp qop infixexp
| - infixexp
| lexp
lexp
→ \ apat1 … apatn -> exp
| let decls in exp
| if exp [;] then exp [;] else exp
| case exp of { alts }
| do { stmts }
| fexp
fexp
→ [fexp] aexp
aexp
→ ( exp )
| …
There just happens to be no case defined in fexp (function application) or aexp (literal expression) that allows an unparenthesised lexp (lambda, let, etc.). I would consider this a bug in the grammar.
Fixing this would also remove the need for the $ typing hack.
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9Years ago, I built GHC from sources with the grammar modified in this sort of way, and it indeed seemed not to break anything (e.g, it could successfully rebuild itself). I especially liked the look of calls which pass arguments to the block, like `withOpenFile path \ handle -> do ...` – Brandon Feb 21 '14 at 04:29
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1@Brandon: I suppose you wouldn't have the time to push this modification as an e.g. `{-# LANGUAGE UnparenthesizedLayoutHeralds #-}` extension? – leftaroundabout Mar 04 '14 at 13:01
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What, _shorter_? Whenever I type `MultiParamTypeClasses` I'm so delighted that Emacs' auto-complete is disabled in "comments", I'd love more of that kind of fun... – leftaroundabout Mar 05 '14 at 00:24
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@leftaroundabout: Solution: type `XMulti`, hit M-/ to `dabbrev-expand` from the compiler error “perhaps you meant `-XMultiParamTypeClasses`”, then delete the `X`. :P – Jon Purdy Mar 05 '14 at 01:23