diff options
Diffstat (limited to 'docs/users_guide/glasgow_exts.rst')
| -rw-r--r-- | docs/users_guide/glasgow_exts.rst | 26 |
1 files changed, 15 insertions, 11 deletions
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst index 3aa9f23bcf..0128f33b0a 100644 --- a/docs/users_guide/glasgow_exts.rst +++ b/docs/users_guide/glasgow_exts.rst @@ -4032,9 +4032,8 @@ Pattern synonyms Allow the definition of pattern synonyms. Pattern synonyms are enabled by the flag :ghc-flag:`-XPatternSynonyms`, which is -required for defining them, but *not* for using them. More information -and examples of view patterns can be found on the -`Wiki page <PatternSynonyms>`. +required for defining them, but *not* for using them. More information and +examples of view patterns can be found on the `Wiki page <PatternSynonyms>`. Pattern synonyms enable giving names to parametrized pattern schemes. They can also be thought of as abstract constructors that don't have a @@ -4301,21 +4300,21 @@ it is assigned a *pattern type* of the form :: pattern P :: CReq => CProv => t1 -> t2 -> ... -> tN -> t -where ⟨CProv⟩ and ⟨CReq⟩ are type contexts, and ⟨t1⟩, ⟨t2⟩, ..., ⟨tN⟩ +where ⟨CReq⟩ and ⟨CProv⟩ are type contexts, and ⟨t1⟩, ⟨t2⟩, ..., ⟨tN⟩ and ⟨t⟩ are types. Notice the unusual form of the type, with two -contexts ⟨CProv⟩ and ⟨CReq⟩: +contexts ⟨CReq⟩ and ⟨CProv⟩: + +- ⟨CReq⟩ are the constraints *required* to match the pattern. - ⟨CProv⟩ are the constraints *made available (provided)* by a successful pattern match. -- ⟨CReq⟩ are the constraints *required* to match the pattern. - For example, consider :: data T a where MkT :: (Show b) => a -> b -> T a - f1 :: (Eq a, Num a) => T a -> String + f1 :: (Num a, Eq a) => T a -> String f1 (MkT 42 x) = show x pattern ExNumPat :: (Num a, Eq a) => (Show b) => b -> T a @@ -4338,8 +4337,13 @@ Exactly the same reasoning applies to ``ExNumPat``: matching against Note also the following points -- In the common case where ``Prov`` is empty, ``()``, it can be omitted - altogether. +- In the common case where ``CProv`` is empty, (i.e., ``()``), it can be + omitted altogether in the above pattern type signature for ``P``. + +- However, if ``CProv`` is non-empty, while ``CReq`` is, the above pattern type + signature for ``P`` must be specified as :: + + P :: () => CProv => t1 -> t2 -> .. -> tN -> t - You may specify an explicit *pattern signature*, as we did for ``ExNumPat`` above, to specify the type of a pattern, just as you can @@ -4400,7 +4404,7 @@ Note also the following points then be rejected. In short, if you want GADT-like behaviour for pattern synonyms, then - (unlike unlike concrete data constructors like ``S1``) you must write + (unlike concrete data constructors like ``S1``) you must write its type with explicit provided equalities. For a concrete data constructor like ``S1`` you can write its type signature as either ``S1 :: Bool -> S Bool`` or ``S1 :: (b~Bool) => Bool -> S b``; the |