hint- Runtime Haskell interpreter (GHC API wrapper)

Portabilitynon-portable (GHC API)
Safe HaskellNone




A Haskell interpreter built on top of the GHC API


The interpreter monad transformer

class (MonadIO m, MonadCatch m) => MonadInterpreter m whereSource


fromSession :: FromSession m aSource

modifySessionRef :: ModifySessionRef m aSource

runGhc :: RunGhc m aSource

Running the interpreter

runInterpreter :: (MonadIO m, MonadCatch m, Functor m) => InterpreterT m a -> m (Either InterpreterError a)Source

Executes the interpreter. Returns Left InterpreterError in case of error.

NB. The underlying ghc will overwrite certain signal handlers (SIGINT, SIGHUP, SIGTERM, SIGQUIT on Posix systems, Ctrl-C handler on Windows). In future versions of hint, this might be controlled by the user.

Interpreter options

data OptionVal m Source


forall a . (Option m a) := a 

get :: MonadInterpreter m => Option m a -> m aSource

Retrieves the value of an option.

set :: MonadInterpreter m => [OptionVal m] -> m ()Source

Use this function to set or modify the value of any option. It is invoked like this:

set [opt1 := val1, opt2 := val2,... optk := valk]

languageExtensions :: MonadInterpreter m => Option m [Extension]Source

Language extensions in use by the interpreter.

Default is: [] (i.e. none, pure Haskell 98)

availableExtensions :: [Extension]Source

List of the extensions known by the interpreter.

glasgowExtensions :: [Extension]Source

Deprecated: glasgowExtensions list is no longer maintained, will be removed soon

List of extensions turned on when the -fglasgow-exts flag is used

data Extension Source

This represents language extensions beyond Haskell 98 that are supported by GHC (it was taken from Cabal's Language.Haskell.Extension)

installedModulesInScope :: MonadInterpreter m => Option m BoolSource

When set to True, every module in every available package is implicitly imported qualified. This is very convenient for interactive evaluation, but can be a problem in sandboxed environments (e.g. unsafePerformIO is in scope).

Default value is True.

Observe that due to limitations in the GHC-API, when set to False, the private symbols in interpreted modules will not be in scope.

searchPath :: MonadInterpreter m => Option m [FilePath]Source

The search path for source files. Observe that every time it is set, it overrides the previous search path. The default is ["."].

Keep in mind that by a limitation in ghc, "." is always in scope.

setUseLanguageExtensions :: MonadInterpreter m => Bool -> m ()Source

Deprecated: Use set [languageExtensions := (ExtendedDefaultRules:glasgowExtensions)] instead.

setInstalledModsAreInScopeQualified :: MonadInterpreter m => Bool -> m ()Source

Deprecated: Use set [installedModulesInScope := b] instead.

Context handling

type ModuleName = StringSource

Module names are _not_ filepaths.

isModuleInterpreted :: MonadInterpreter m => ModuleName -> m BoolSource

Returns True if the module was interpreted.

loadModules :: MonadInterpreter m => [String] -> m ()Source

Tries to load all the requested modules from their source file. Modules my be indicated by their ModuleName (e.g. "My.Module") or by the full path to its source file.

The interpreter is reset both before loading the modules and in the event of an error.

getLoadedModules :: MonadInterpreter m => m [ModuleName]Source

Returns the list of modules loaded with loadModules.

setTopLevelModules :: MonadInterpreter m => [ModuleName] -> m ()Source

Sets the modules whose context is used during evaluation. All bindings of these modules are in scope, not only those exported.

Modules must be interpreted to use this function.

setImports :: MonadInterpreter m => [ModuleName] -> m ()Source

Sets the modules whose exports must be in context.

Warning: setImports and setImportsQ are mutually exclusive. If you have a list of modules to be used qualified and another list unqualified, then you need to do something like

  setImportsQ ((zip unqualified $ repeat Nothing) ++ qualifieds)

setImportsQ :: MonadInterpreter m => [(ModuleName, Maybe String)] -> m ()Source

Sets the modules whose exports must be in context; some of them may be qualified. E.g.:

setImportsQ [(Prelude, Nothing), (Data.Map, Just M)].

Here, map will refer to Prelude.map and M.map to Data.Map.map.

reset :: MonadInterpreter m => m ()Source

All imported modules are cleared from the context, and loaded modules are unloaded. It is similar to a :load in GHCi, but observe that not even the Prelude will be in context after a reset.

Module querying

type Id = StringSource

An Id for a class, a type constructor, a data constructor, a binding, etc

getModuleExports :: MonadInterpreter m => ModuleName -> m [ModuleElem]Source

Gets an abstract representation of all the entities exported by the module. It is similar to the :browse command in GHCi.


Please note below that annotations are an experimental feature in GHC HEAD. In the snippets below we use 'LBRACE' and 'RBRACE' to mean '{' and '}' respectively. We cannot put the pragmas inline in the code since GHC scarfs them up.

getModuleAnnotations :: (Data a, MonadInterpreter m) => a -> String -> m [a]Source

Get the annotations associated with a particular module.

For example, given:

   RBRACE-# ANN module (1 :: Int) #-LBRACE
   module SomeModule(g, h) where

Then after using loadModule to load SomeModule into scope:

   x <- getModuleAnnotations (as :: Int) SomeModule
   liftIO $ print x
   -- result is [1]

getValAnnotations :: (Data a, MonadInterpreter m) => a -> String -> m [a]Source

Get the annotations associated with a particular function

For example, given:

   module SomeModule(g, h) where

LBRACE-# ANN g (Just 1 :: Maybe Int) #-RBRACE
   g = f [f]

LBRACE-# ANN h (Just 2 :: Maybe Int) #-RBRACE
   h = f

Then after using loadModule to bring SomeModule into scope:

   x <- liftM concat $ mapM (getValAnnotations (as :: Maybe Int)) ["g","h"]
   liftIO $ print x
   -- result is [Just 2, Just 1]

This can also work on data constructors and types with annotations.

Type inference

typeOf :: MonadInterpreter m => String -> m StringSource

Returns a string representation of the type of the expression.

typeChecks :: MonadInterpreter m => String -> m BoolSource

Tests if the expression type checks.

kindOf :: MonadInterpreter m => String -> m StringSource

Returns a string representation of the kind of the type expression.


interpret :: (MonadInterpreter m, Typeable a) => String -> a -> m aSource

Evaluates an expression, given a witness for its monomorphic type.

as :: Typeable a => aSource

Convenience functions to be used with interpret to provide witnesses. Example:

  • interpret "head [True,False]" (as :: Bool)
  • interpret "head $ map show [True,False]" infer >>= flip interpret (as :: Bool)

infer :: Typeable a => aSource

Convenience functions to be used with interpret to provide witnesses. Example:

  • interpret "head [True,False]" (as :: Bool)
  • interpret "head $ map show [True,False]" infer >>= flip interpret (as :: Bool)

eval :: MonadInterpreter m => String -> m StringSource

eval expr will evaluate show expr. It will succeed only if expr has type t and there is a Show instance for t.

Error handling

data InterpreterError Source


UnknownError String 
WontCompile [GhcError] 
NotAllowed String 
GhcException String

GhcExceptions from the underlying GHC API are caught and rethrown as this.

newtype GhcError Source




errMsg :: String


data MultipleInstancesNotAllowed Source

The installed version of ghc is not thread-safe. This exception is thrown whenever you try to execute runInterpreter while another instance is already running.


ghcVersion :: IntSource

Version of the underlying ghc api. Values are:

  • 606 for GHC 6.6.x
  • 608 for GHC 6.8.x
  • 610 for GHC 6.10.x
  • etc...

parens :: String -> StringSource

Conceptually, parens s = "(" ++ s ++ ")", where s is any valid haskell expression. In practice, it is harder than this. Observe that if s ends with a trailing comment, then parens s would be a malformed expression. The straightforward solution for this is to put the closing parenthesis in a different line. However, now we are messing with the layout rules and we don't know where s is going to be used! Solution: parens s = "(let {foo =n" ++ s ++ "\n ;} in foo)" where foo does not occur in s