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Diffstat (limited to 'docs/users_guide/using-optimisation.rst')
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diff --git a/docs/users_guide/using-optimisation.rst b/docs/users_guide/using-optimisation.rst index 48c1e6f3bb..123d74bc8f 100644 --- a/docs/users_guide/using-optimisation.rst +++ b/docs/users_guide/using-optimisation.rst @@ -599,6 +599,37 @@ list. which they are called in this module. Note that specialisation must be enabled (by ``-fspecialise``) for this to have any effect. +.. ghc-flag:: -fsolve-constant-dicts + + :default on + + When solving constraints, try to eagerly solve + super classes using availible dictionaries. + + For example:: + + class M a b where m :: a -> b + + type C a b = (Num a, M a b) + + f :: C Int b => b -> Int -> Int + f _ x = x + 1 + + The body of `f` requires a `Num Int` instance. We could solve this + constraint from the context because we have `C Int b` and that provides us + a + solution for `Num Int`. However, we can often produce much better code + by directly solving for an availible `Num Int` dictionary we might have at + hand. This removes potentially many layers of indirection and crucially + allows other optimisations to fire as the dictionary will be statically + known and selector functions can be inlined. + + The optimisation also works for GADTs which bind dictionaries. If we + statically know which class dictionary we need then we will solve it + directly rather than indirectly using the one passed in at run time. + + + .. ghc-flag:: -fstatic-argument-transformation Turn on the static argument transformation, which turns a recursive |