GHC 7.10.x Migration Guide

This guide summarises the changes you may need to make to your code to migrate from GHC 7.8 to GHC 7.10.

Compiler changes

Inferred type-signatures now may require to enable FlexibleContexts, GADTs, or TypeFamilies

Basically GHC has become stricter and needs to explicitly enable language extensions in some cases that weren't needed previously. GHC will tell you which extension to enable. See also #8883 for details.

Example breakages:

Library changes


The major changes include the implementation of the Functor/Applicative/Monad-Proposal as well as the Foldable/Traversable-Proposal

GHC says No instance for (Applicative ...)

If GHC complains that

    No instance for (Applicative Foo) arising from the superclasses of an instance declaration
    In the instance declaration for ‘Monad Foo’

then an easy way to fix this error is by defining an Applicative (and possibly a Functor) instance:

instance Functor Foo where
    fmap  = liftM
    -- or alternatively:
    --  fmap = m >>= pure . f

instance Applicative Foo where
    -- NB: DO NOT USE `pure = return`
    pure  = {- move the definition of `return` from the `Monad` instance here -}

    (<*>) = ap  {- defined in Control.Monad -}
    -- or alternatively:
    --  f1 <*> f2 = f1 >>= \v1 -> f2 >>= (pure . v1)

    -- NB: DO NOT USE `(*>) = (>>)`
    (*>) = {- move the definition of `>>` from the `Monad` instance here -}

instance Monad Foo where
    return = pure {- definition moved to `Applicative(pure)` -}

    (>>) = (*>) {- definition moved to `Applicative((*>))` -}

    {- ...retain other previous definitions... -}

Starting with GHC 7.10, defining Monad(return) is optional (with the default implementation return = pure). For code not required to be compatible with GHC versions prior to GHC 7.10 it's recommended to omit the explicit return definition.

Moreover, in order to avoid surprising performance issues with Traversable, (>>) should be defined in terms of (*>), and with (*>) optionally being overridden by an optimised implementation.

See also Proposal/MonadOfNoReturn

For more details see AMP Proposal: Future-proofing current code.

GHC says No instance for (Alternative ...)

A side-effect of the AMP is that Alternative became a super-class of MonadPlus. The easy remedy:

instance Alternative Foo where
    (<|>) = mplus
    empty = mzero

If pre-AMP compatibility is not a requirement, it's better to define MonadPlus in terms of Alternative as this corresponds to the superclass relationship and allows to omit the method definitions in MonadPlus, i.e.

instance Alternative Foo where
   empty = {- move definition of `mzero` here -}
   (<|>) = {- move definition of `mplus` here -}

instance MonadPlus Foo -- NB: starting with `base-4.8`: no need to override methods here!

GHC says No instance for (Foldable ...) arising from the use of...

Some functions in the Prelude were generalised to Foldables in the implementation of the FTP. This can lead to type-inference problems for code making use of -XOverloadedStrings, in which code like e.g.

{-# LANGUAGE OverloadedStrings #-}

main :: IO ()
main = print ('I' `elem` "team")

will let GHC spit out the intimidating complaint that

    No instance for (Foldable t0) arising from a use of ‘elem’
    The type variable ‘t0’ is ambiguous
    Note: there are several potential instances:
      instance Foldable (Either a) -- Defined in ‘Data.Foldable’
      instance Foldable Data.Proxy.Proxy -- Defined in ‘Data.Foldable’
      instance GHC.Arr.Ix i => Foldable (GHC.Arr.Array i) -- Defined in ‘Data.Foldable’ three others
    In the first argument of ‘print’, namely ‘('I' `elem` "team")’
    In the expression: print ('I' `elem` "team")
    In an equation for ‘main’: main = print ('I' `elem` "team")

    No instance for (Data.String.IsString (t0 Char)) arising from the literal ‘"team"’
    The type variable ‘t0’ is ambiguous
    Note: there is a potential instance available: instance Data.String.IsString [Char] -- Defined in ‘Data.String’
    In the second argument of ‘elem’, namely ‘"team"’
    In the first argument of ‘print’, namely ‘('I' `elem` "team")’
    In the expression: print ('I' `elem` "team")

This can be fixed by helping GHC's type-inference to pin down "team" to [Char], either by adding a :: String to the string literal, or (for short literals) by using list-syntax (i.e. ['t','e','a','m']).

GHC says The import of ... is redundant

With base-4.8, the Prelude re-exports a few more entities from modules such as Data.Monoid and Control.Applicative. For example, the following module

module Foo (Int, Word, Monoid(..)) where

import Data.Monoid
import Data.Word
-- import Prelude

will result in two redundant import warnings with GHC 7.10.1 (under -Wall):

foo.hs:3:1: Warning:
    The import of ‘Data.Monoid’ is redundant
      except perhaps to import instances from ‘Data.Monoid’
    To import instances alone, use: import Data.Monoid()

foo.hs:4:1: Warning:
    The import of ‘Data.Word’ is redundant
      except perhaps to import instances from ‘Data.Word’
    To import instances alone, use: import Data.Word()

This import Prelude-trick, however, doesn't work when explicit import lists are used. In that case, qualified imports can be used if -XCPP needs to be avoided at all costs.

One obvious way to silence such warnings is use -XCPP. However, there's also a less-known trick/hack to moving the implicit import Prelude into an explicit one after all base-imports. In the code example above just de-comment the -- import Prelude line.

A more robust way (that works when the trick above doesn't work) to convince GHC without -XCPP to suppress redundancy-warnings is by referencing a module-prefixed entity form the respective module at least once. In the example below, pure is referenced just once via A.pure, thereby marking the import essential:

import Control.Applicative as A (Applicative(..))

data Maybe' a = Nothing' | Just' a

instance Functor Maybe' where
    fmap f (Just' v) = Just' (f v)
    fmap _ Nothing'  = Nothing'

instance Applicative Maybe' where
    pure = Just'
    f1 <*> f2   = f1 >>= \v1 -> f2 >>= (pure . v1)

instance Monad Maybe' where
    Nothing' >>= _  = Nothing'
    Just' x  >>= f  = f x

    return = A.pure -- NB: This definition is redundant since GHC 7.10/base-4.8

deepseq- (& deepseq-

The default method implementation has been changed, so instances defined via

instance NFData Foo

will now result in the following compile error (unless a Generic instance happens to exist):

    No instance for (GHC.Generics.Generic Foo) arising from a use of ‘Control.DeepSeq.$gdmrnf’
    In the expression: Control.DeepSeq.$gdmrnf
    In an equation for ‘rnf’: rnf = Control.DeepSeq.$gdmrnf
    In the instance declaration for ‘NFData Foo’

The easiest way make code work again with deepseq- which was relying on the pre-1.4 default method implementation of rnf is to explicitly define rnf like so

instance NFData Foo where rnf x = seq x ()

instead of relying on the default method implementation.

For more details (and how to use the new facilities) see Haddock documentation of hackage:deepseq


The internal representation for Integers has changed. Code only referencing the S# constructor should not be affected. The J# constructor however has been replaced by two new Jp# and Jn# constructors (see below).

Old representation:

data Integer
   = S# Int#            -- ^ \"small\" integers fitting into an 'Int#'
   | J# Int# ByteArray# -- ^ \"big\" integers (legal to be used to represent 'Int#'-range)

New representation:

data Integer 
  = S# Int#      -- ^ iff value in @[minBound::'Int', maxBound::'Int']@ range
  | Jp# !BigNat  -- ^ iff value in @]maxBound::'Int', +inf[@ range
  | Jn# !BigNat  -- ^ iff value in @]-inf, minBound::'Int'[@ range

Please note that the new representation is now stricter regarding internal invariants (note the if-and-only-ifs iffs in the comments above). For more details, see


There are a number of API changes in this release of Template Haskell.

  • The Pred type (recall type Cxt = [Pred]) is now a synonym of Type, to allow TH to work with the ConstraintKinds extension. There is thus a new EqualityT constructor of Type that represents ~. The classP and equalP functions from TH.Lib remain, but are deprecated.
  • There are two new Dec forms: StandaloneDerivD and DefaultSigD. Check the relevant documentation for more details.


The major breaking change in hackage:time- is not requiring old-locale anymore and providing its own locale-type.

See for an example how to make code compatible with both, time < 1.5 and time >= 1.5, or use


In, one could derive Data.Array.Unboxed instances for newtype Int's, Word64's etc.:

newtype N = MkN Word64
  deriving (IArray UArray)

After changed the UArray role to nominal, one has to manually create instances (see for an example).

Last modified 3 months ago Last modified on Dec 6, 2016 11:44:39 AM