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| author | Simon Peyton Jones <simonpj@microsoft.com> | 2016-04-18 15:14:40 +0100 |
|---|---|---|
| committer | Simon Peyton Jones <simonpj@microsoft.com> | 2016-04-19 08:38:47 +0100 |
| commit | 8136a5cbfcd24647f897a2fae9fcbda0b1624035 (patch) | |
| tree | 8c5c8100cbe1b6552d5fbdd6819c9d4417fac48f /testsuite/tests/polykinds | |
| parent | 17eb2419c42c70d7436b6b8cff0cef705353bb4e (diff) | |
| download | haskell-8136a5cbfcd24647f897a2fae9fcbda0b1624035.tar.gz | |
Tighten checking for associated type instances
This patch finishes off Trac #11450. Following debate on that ticket,
the patch tightens up the rules for what the instances of an
associated type can look like. Now they must match the instance
header exactly. Eg
class C a b where
type T a x b
With this class decl, if we have an instance decl
instance C ty1 ty2 where ...
then the type instance must look like
type T ty1 v ty2 = ...
with exactly
- 'ty1' for 'a'
- 'ty2' for 'b', and
- a variable for 'x'
For example:
instance C [p] Int
type T [p] y Int = (p,y,y)
Previously we allowed multiple instance equations and now, in effect,
we don't since they would all overlap. If you want multiple cases,
use an auxiliary type family.
This is consistent with the treatment of generic-default instances,
and the user manual always said "WARNING: this facility (multiple
instance equations may be withdrawn in the future".
I also improved error messages, and did other minor refactoring.
Diffstat (limited to 'testsuite/tests/polykinds')
| -rw-r--r-- | testsuite/tests/polykinds/T10570.stderr | 14 |
1 files changed, 7 insertions, 7 deletions
diff --git a/testsuite/tests/polykinds/T10570.stderr b/testsuite/tests/polykinds/T10570.stderr index f40ed10ce4..3c91db5cfb 100644 --- a/testsuite/tests/polykinds/T10570.stderr +++ b/testsuite/tests/polykinds/T10570.stderr @@ -1,9 +1,9 @@ T10570.hs:10:10: error: - Illegal instance declaration for ‘ConsByIdx2 Int a Proxy cls’ - The coverage condition fails in class ‘ConsByIdx2’ - for functional dependency: ‘x -> m’ - Reason: lhs type ‘Int’ does not determine rhs type ‘Proxy’ - Un-determined variable: k - (Use -fprint-explicit-kinds to see the kind variables in the types) - In the instance declaration for ‘ConsByIdx2 Int a Proxy cls’ + • Illegal instance declaration for ‘ConsByIdx2 Int a Proxy cls’ + The coverage condition fails in class ‘ConsByIdx2’ + for functional dependency: ‘x -> m’ + Reason: lhs type ‘Int’ does not determine rhs type ‘Proxy’ + Un-determined variable: k + Use -fprint-explicit-kinds to see the kind arguments + • In the instance declaration for ‘ConsByIdx2 Int a Proxy cls’ |