Using :paste in :paste mode crashes the repl

[natefaubion]

It's a little silly, but we could maybe recover from it I think.

> :paste
… :paste
…
purs: An internal error occurred during compilation: "handleCommand: unexpected command"
Please report this at https://github.com/purescript/purescript/issues
CallStack (from HasCallStack):
  error, called at src/Language/PureScript/Crash.hs:24:3 in purescript-0.12.1-Cp6zGLfC9uqBgP73eCu6tr:Language.PureScript.Crash
  internalError, called at src/Language/PureScript/Interactive.hs:115:49 in purescript-0.12.1-Cp6zGLfC9uqBgP73eCu6tr:Language.PureScript.Interactive

[hdgarrood]

This is perhaps a sign that we should just be using proper Parsec parsers throughout Language.PureScript.Interactive.Parser, rather than the current mixture of parsec parsers and String -> Either String a functions.

[hdgarrood]

Actually I take that back, the way we're using Parsec doesn't seem to be related to this.

Having looked into it a little more I wonder if the issue is that we shouldn't be allowing any command inside :paste mode. I don't think

> :paste
… :k Int
… 
Type

makes a lot of sense, for instance. Looking at the Command data type,

data Command
  -- | A purescript expression
  = Expression P.Expr
  -- | Show the help (ie, list of directives)
  | ShowHelp
  -- | Import a module from a loaded file
  | Import ImportedModule
  -- | Browse a module
  | BrowseModule P.ModuleName
  -- | Exit PSCI
  | QuitPSCi
  -- | Reload all the imported modules of the REPL
  | ReloadState
  -- | Clear the state of the REPL
  | ClearState
  -- | Add some declarations to the current evaluation context
  | Decls [P.Declaration]
  -- | Find the type of an expression
  | TypeOf P.Expr
  -- | Find the kind of an expression
  | KindOf P.SourceType
  -- | Shows information about the current state of the REPL
  | ShowInfo ReplQuery
  -- | Paste multiple lines
  | PasteLines
  -- | Return auto-completion output as if pressing <tab>
  | CompleteStr String
  -- | Set the interactive printing function
  | SetInteractivePrint (P.ModuleName, P.Ident)
  deriving Show

I think the only constructors which make sense inside a :paste would be Expression, Decls, and Import. How about disallowing everything else inside :paste? We can probably give more helpful syntax errors that way, too.