Repository : ssh://darcs.haskell.org//srv/darcs/ghc On branch : master
http://hackage.haskell.org/trac/ghc/changeset/215cf423abed67a2827e0ee1ec95e8fcdfe0c8df >--------------------------------------------------------------- commit 215cf423abed67a2827e0ee1ec95e8fcdfe0c8df Author: Simon Peyton Jones <simo...@microsoft.com> Date: Tue Jan 1 23:13:20 2013 +0000 Make the comments about SingI and EvLit match current reality See Note [SingI and EvLit] in TcEvidence. >--------------------------------------------------------------- compiler/prelude/PrelNames.lhs | 1 + compiler/typecheck/TcEvidence.lhs | 29 ++++++++++++++--------------- 2 files changed, 15 insertions(+), 15 deletions(-) diff --git a/compiler/prelude/PrelNames.lhs b/compiler/prelude/PrelNames.lhs index 5d8dc3c..4bb0e54 100644 --- a/compiler/prelude/PrelNames.lhs +++ b/compiler/prelude/PrelNames.lhs @@ -1216,6 +1216,7 @@ datatypeClassKey = mkPreludeClassUnique 39 constructorClassKey = mkPreludeClassUnique 40 selectorClassKey = mkPreludeClassUnique 41 +-- SingI: see Note [SingI and EvLit] in TcEvidence singIClassNameKey, typeNatLeqClassNameKey :: Unique singIClassNameKey = mkPreludeClassUnique 42 typeNatLeqClassNameKey = mkPreludeClassUnique 43 diff --git a/compiler/typecheck/TcEvidence.lhs b/compiler/typecheck/TcEvidence.lhs index fe38a07..452a769 100644 --- a/compiler/typecheck/TcEvidence.lhs +++ b/compiler/typecheck/TcEvidence.lhs @@ -496,7 +496,7 @@ data EvTerm -- selector Id. We count up from _0_ | EvLit EvLit -- Dictionary for class "SingI" for type lits. - -- Note [EvLit] + -- Note [SingI and EvLit] deriving( Data.Data, Data.Typeable) @@ -550,27 +550,26 @@ Conclusion: a new wanted coercion variable should be made mutable. from super classes will be "given" and hence rigid] -Note [EvLit] -~~~~~~~~~~~~ +Note [SingI and EvLit] +~~~~~~~~~~~~~~~~~~~~~~ A part of the type-level literals implementation is the class "SingI", which provides a "smart" constructor for defining singleton values. +Here is the key stuff from GHC.TypeLits -newtype Sing n = Sing (SingRep n) + class SingI n where + sing :: Sing n -class SingI n where - sing :: Sing n - -type family SingRep a -type instance SingRep (a :: Nat) = Integer -type instance SingRep (a :: Symbol) = String + data family Sing (n::k) + newtype instance Sing (n :: Nat) = SNat Integer + newtype instance Sing (s :: Symbol) = SSym String Conceptually, this class has infinitely many instances: -instance Sing 0 where sing = Sing 0 -instance Sing 1 where sing = Sing 1 -instance Sing 2 where sing = Sing 2 -instance Sing "hello" where sing = Sing "hello" -... + instance Sing 0 where sing = SNat 0 + instance Sing 1 where sing = SNat 1 + instance Sing 2 where sing = SNat 2 + instance Sing "hello" where sing = SSym "hello" + ... In practice, we solve "SingI" predicates in the type-checker because we can't have infinately many instances. The evidence (aka "dictionary") _______________________________________________ Cvs-ghc mailing list Cvs-ghc@haskell.org http://www.haskell.org/mailman/listinfo/cvs-ghc
