GHCi: add :seti, for options that apply only at the prompt (#3217)
GHCi now maintains two DynFlags: one that applies to whole modules loaded with :load, and one that applies to things typed at the prompt (expressions, statements, declarations, commands). The :set command modifies both DynFlags. This is for backwards compatibility: users won't notice any difference. The :seti command applies only to the interactive DynFlags. Additionally, I made a few changes to ":set" (with no arguments): * Now it only prints out options that differ from the defaults, rather than the whole list. * There is a new variant, ":set -a" to print out all options (the old behaviour). * It also prints out language options. e.g. Prelude> :set options currently set: none. base language is: Haskell2010 with the following modifiers: -XNoDatatypeContexts -XNondecreasingIndentation GHCi-specific dynamic flag settings: other dynamic, non-language, flag settings: -fimplicit-import-qualified warning settings: ":seti" (with no arguments) does the same as ":set", but for the interactive options. It also has the "-a" option. The interactive DynFlags are kept in the InteractiveContext, and copied into the HscEnv at the appropriate points (all in HscMain). There are some new GHC API operations: -- | Set the 'DynFlags' used to evaluate interactive expressions. setInteractiveDynFlags :: GhcMonad m => DynFlags -> m () -- | Get the 'DynFlags' used to evaluate interactive expressions. getInteractiveDynFlags :: GhcMonad m => m DynFlags -- | Sets the program 'DynFlags'. setProgramDynFlags :: GhcMonad m => DynFlags -> m [PackageId] -- | Returns the program 'DynFlags'. getProgramDynFlags :: GhcMonad m => m DynFlags Note I have not completed the whole of the plan outlined in #3217 yet: when in the context of a loaded module we don't take the interactive DynFlags from that module. That needs some more refactoring and thinking about, because we'll need to save and restore the original interactive DynFlags. This solves the immediate problem that people are having with the new flag checking in 7.4.1, because now it is possible to set language options in ~/.ghci that do not affect loaded modules and thereby cause recompilation.
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