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| author | Richard Eisenberg <eir@cis.upenn.edu> | 2014-11-21 10:51:38 -0500 |
|---|---|---|
| committer | Richard Eisenberg <eir@cis.upenn.edu> | 2014-11-21 11:15:49 -0500 |
| commit | cfa574cea30b411080de5d641309bdf135ed9be5 (patch) | |
| tree | 96bd229fc0a0acee2f4bad4df479b8959d95b9d8 /docs | |
| parent | bc05354949dce9d3b56353d3310eb9804d4e17f5 (diff) | |
| download | haskell-cfa574cea30b411080de5d641309bdf135ed9be5.tar.gz | |
Update manual for pattern splices (#1476)
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 55 |
1 files changed, 42 insertions, 13 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index a21e677943..f73a1d9980 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -8785,25 +8785,54 @@ h z = z-1 <listitem> <para> - Binders are lexically scoped. For example, consider the - following code, where a value <literal>g</literal> of type - <literal>Bool -> Q Pat</literal> is in scope, having been - imported from another module + Outermost pattern splices may bind variables. By "outermost" here, we refer to + a pattern splice that occurs outside of any quotation brackets. For example, + <programlisting> -y :: Int -y = 7 +mkPat :: Bool -> Q Pat +mkPat True = [p| (x, y) |] +mkPat False = [p| (y, x) |] -f :: Int -> Int -> Int -f n = \ $(g True) -> y+n +-- in another module: +foo :: (Char, String) -> String +foo $(mkPat True) = x : y + +bar :: (String, Char) -> String +bar $(mkPat False) = x : y +</programlisting> + </para> + </listitem> + + + <listitem> + <para> + Nested pattern splices do <emphasis>not</emphasis> bind variables. + By "nested" here, we refer to a pattern splice occurring within a + quotation bracket. Continuing the example from the last bullet: + +<programlisting> +baz :: Bool -> Q Exp +baz b = [| quux $(mkPat b) = x + y |] </programlisting> - The <literal>y</literal> in the right-hand side of - <literal>f</literal> refers to the top-level <literal>y = - 7</literal>, even if the pattern splice <literal>$(g - n)</literal> also generates a binder <literal>y</literal>. + + would fail with <literal>x</literal> and <literal>y</literal> + being out of scope. </para> <para> - Note that a pattern quasiquoter <emphasis>may</emphasis> + The difference in treatment of outermost and nested pattern splices is + because outermost splices are run at compile time. GHC can then use + the result of running the splice when analyzing the expressions within + the pattern's scope. Nested splices, on the other hand, are <emphasis>not</emphasis> + run at compile time; they are run when the bracket is spliced in, sometime later. + Since nested pattern splices may refer to local variables, there is no way for GHC + to know, at splice compile time, what variables are bound, so it binds none. + </para> + </listitem> + + <listitem> + <para> + A pattern quasiquoter <emphasis>may</emphasis> generate binders that scope over the right-hand side of a definition because these binders are in scope lexically. For example, given a quasiquoter <literal>haskell</literal> that |