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author | Ryan Scott <ryan.gl.scott@gmail.com> | 2021-04-13 18:17:19 -0400 |
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committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2021-04-29 17:27:54 -0400 |
commit | 5981ac7d83810d26d74a07bbc452a0be278de66c (patch) | |
tree | 6681ca9ff678352669f763666e7734ed22143fc6 /libraries/ghc-prim | |
parent | 7d18e1bace3f3a85eae177654690d91b688c0e8f (diff) | |
download | haskell-5981ac7d83810d26d74a07bbc452a0be278de66c.tar.gz |
Redesign withDict (formerly magicDict)
This gives a more precise type signature to `magicDict` as proposed in #16646.
In addition, this replaces the constant-folding rule for `magicDict` in
`GHC.Core.Opt.ConstantFold` with a special case in the desugarer in
`GHC.HsToCore.Expr.dsHsWrapped`. I have also renamed `magicDict` to `withDict`
in light of the discussion in
https://mail.haskell.org/pipermail/ghc-devs/2021-April/019833.html.
All of this has the following benefits:
* `withDict` is now more type safe than before. Moreover, if a user applies
`withDict` at an incorrect type, the special-casing in `dsHsWrapped` will
now throw an error message indicating what the user did incorrectly.
* `withDict` can now work with classes that have multiple type arguments, such
as `Typeable @k a`. This means that `Data.Typeable.Internal.withTypeable` can
now be implemented in terms of `withDict`.
* Since the special-casing for `withDict` no longer needs to match on the
structure of the expression passed as an argument to `withDict`, it no
longer cares about the presence or absence of `Tick`s. In effect, this
obsoletes the fix for #19667.
The new `T16646` test case demonstrates the new version of `withDict` in
action, both in terms of `base` functions defined in terms of `withDict`
as well as in terms of functions from the `reflection` and `singletons`
libraries. The `T16646Fail` test case demonstrates the error message that GHC
throws when `withDict` is applied incorrectly.
This fixes #16646. By adding more tests for `withDict`, this also
fixes #19673 as a side effect.
Diffstat (limited to 'libraries/ghc-prim')
-rw-r--r-- | libraries/ghc-prim/GHC/Magic/Dict.hs | 44 | ||||
-rw-r--r-- | libraries/ghc-prim/changelog.md | 26 | ||||
-rw-r--r-- | libraries/ghc-prim/ghc-prim.cabal | 1 |
3 files changed, 69 insertions, 2 deletions
diff --git a/libraries/ghc-prim/GHC/Magic/Dict.hs b/libraries/ghc-prim/GHC/Magic/Dict.hs new file mode 100644 index 0000000000..12861db568 --- /dev/null +++ b/libraries/ghc-prim/GHC/Magic/Dict.hs @@ -0,0 +1,44 @@ +{-# LANGUAGE ConstraintKinds #-} +{-# LANGUAGE DataKinds #-} +{-# LANGUAGE ImpredicativeTypes #-} +{-# LANGUAGE MagicHash #-} +{-# LANGUAGE NoImplicitPrelude #-} +{-# LANGUAGE PolyKinds #-} +{-# LANGUAGE RankNTypes #-} +{-# LANGUAGE Unsafe #-} + +----------------------------------------------------------------------------- +-- | +-- Module : GHC.Magic.Dict +-- Copyright : (c) The University of Glasgow 2009 +-- License : see libraries/ghc-prim/LICENSE +-- +-- Maintainer : cvs-ghc@haskell.org +-- Stability : internal +-- Portability : non-portable (GHC Extensions) +-- +-- Defines the 'withDict' function. For more information, see +-- @Note [withDict]@ in "GHC.HsToCore.Expr" in GHC. +-- The definition of 'withDict' is located in a separate module from +-- "GHC.Magic" because 'withDict' is @Unsafe@ (it threatens type class +-- coherence) while "GHC.Magic" is @Trustworthy@. +-- +-- Use "GHC.Exts" from the @base@ package instead of importing this +-- module directly. +-- +----------------------------------------------------------------------------- + +module GHC.Magic.Dict (withDict) where + +import GHC.Prim.Panic (panicError) +import GHC.Types (RuntimeRep, TYPE) + +-- | @'withDict' d f@ provides a way to call a type-class–overloaded function +-- @f@ by applying it to the supplied dictionary @d@. +-- +-- 'withDict' can only be used if the type class has a single method with no +-- superclasses. For more (important) details on how this works, see +-- @Note [withDict]@ in "GHC.HsToCore.Expr" in GHC. +withDict :: forall {rr :: RuntimeRep} st dt (r :: TYPE rr). st -> (dt => r) -> r +{-# NOINLINE withDict #-} +withDict = panicError "Non-rewritten withDict"# diff --git a/libraries/ghc-prim/changelog.md b/libraries/ghc-prim/changelog.md index 6c7723068f..1ce61e2e61 100644 --- a/libraries/ghc-prim/changelog.md +++ b/libraries/ghc-prim/changelog.md @@ -1,3 +1,25 @@ +## next (edit as necessary) + +- `magicDict` has been renamed to `withDict` and is now defined in + `GHC.Magic.Dict` instead of `GHC.Prim`. `withDict` now has the type: + + ``` + withDict :: forall {rr :: RuntimeRep} st dt (r :: TYPE rr). st -> (dt => r) -> r + ``` + + Unlike `magicDict`, `withDict` can be used without defining an + intermediate data type. For example, the `withTypeable` function from the + `Data.Typeable` module can now be defined as: + + ``` + withTypeable :: forall k (a :: k) rep (r :: TYPE rep). () + => TypeRep a -> (Typeable a => r) -> r + withTypeable rep k = withDict @(TypeRep a) @(Typeable a) rep k + ``` + + Note that the explicit type applications are required, as the call to + `withDict` would be ambiguous otherwise. + ## 0.8.0 (edit as necessary) - Change array access primops to use type with size maxing the element size: @@ -23,7 +45,7 @@ - Add known-key `cstringLength#` to `GHC.CString`. This is just the C function `strlen`, but a built-in rewrite rule allows GHC to compute the result at compile time when the argument is known. - + - In order to support unicode better the following functions in `GHC.CString` gained UTF8 counterparts: @@ -47,7 +69,7 @@ atomicCasAddrAddr# :: Addr# -> Addr# -> Addr# -> State# s -> (# State# s, Addr# #) atomicCasWordAddr# :: Addr# -> Word# -> Word# -> State# s -> (# State# s, Word# #) -- Add an explicit fixity for `(~)` and `(~~)`: +- Add an explicit fixity for `(~)` and `(~~)`: infix 4 ~, ~~ diff --git a/libraries/ghc-prim/ghc-prim.cabal b/libraries/ghc-prim/ghc-prim.cabal index 05fd60f09a..61840021c1 100644 --- a/libraries/ghc-prim/ghc-prim.cabal +++ b/libraries/ghc-prim/ghc-prim.cabal @@ -45,6 +45,7 @@ Library GHC.Debug GHC.IntWord64 GHC.Magic + GHC.Magic.Dict GHC.Prim.Ext GHC.Prim.Panic GHC.Prim.Exception |