Commit 536852b1 authored by Isaac Dupree's avatar Isaac Dupree
Browse files

Add a warning flag for when the Prelude is implicitly imported (trac #1317)

GHC already determines all the implicit (Prelude) imports,
so we just need to check whether there are any of those,
for each module being compiled.
parent 53167b89
......@@ -139,6 +139,7 @@ data DynFlag
| Opt_WarnIsError -- -Werror; makes warnings fatal
| Opt_WarnDuplicateExports
| Opt_WarnHiShadows
| Opt_WarnImplicitPrelude
| Opt_WarnIncompletePatterns
| Opt_WarnIncompletePatternsRecUpd
| Opt_WarnMissingFields
......@@ -1024,6 +1025,7 @@ dynamic_flags = [
fFlags = [
( "warn-duplicate-exports", Opt_WarnDuplicateExports ),
( "warn-hi-shadowing", Opt_WarnHiShadows ),
( "warn-implicit-prelude", Opt_WarnImplicitPrelude ),
( "warn-incomplete-patterns", Opt_WarnIncompletePatterns ),
( "warn-incomplete-record-updates", Opt_WarnIncompletePatternsRecUpd ),
( "warn-missing-fields", Opt_WarnMissingFields ),
......
......@@ -69,6 +69,10 @@ rnImports imports
(source, ordinary) = partition is_source_import imports
is_source_import (L _ (ImportDecl _ is_boot _ _ _)) = is_boot
ifOptM Opt_WarnImplicitPrelude (
when (notNull prel_imports) $ addWarn (implicitPreludeWarn)
)
stuff1 <- mapM (rnImportDecl this_mod) (prel_imports ++ ordinary)
stuff2 <- mapM (rnImportDecl this_mod) source
let (decls, rdr_env, imp_avails) = combine (stuff1 ++ stuff2)
......@@ -1355,4 +1359,7 @@ nullModuleExport mod
moduleDeprec mod txt
= sep [ ptext SLIT("Module") <+> quotes (ppr mod) <+> ptext SLIT("is deprecated:"),
nest 4 (ppr txt) ]
implicitPreludeWarn
= ptext SLIT("Module `Prelude' implicitly imported")
\end{code}
......@@ -733,7 +733,7 @@
</row>
<row>
<entry><option>-Wall</option></entry>
<entry>enable all warnings</entry>
<entry>enable almost all warnings (details in <xref linkend="options-sanity"/>)</entry>
<entry>dynamic</entry>
<entry><option>-w</option></entry>
</row>
......@@ -766,6 +766,13 @@
<entry><option>-fno-warn-hi-shadowing</option></entry>
</row>
<row>
<entry><option>-fwarn-implicit-prelude</option></entry>
<entry>warn when the Prelude is implicitly imported</entry>
<entry>dynamic</entry>
<entry><option>-fno-warn-implicit-prelude</option></entry>
</row>
<row>
<entry><option>-fwarn-incomplete-patterns</option></entry>
<entry>warn when a pattern match could fail</entry>
......
......@@ -848,6 +848,7 @@ ghc -c Foo.hs</screen>
<listitem><option>-fwarn-tabs</option></listitem>
<listitem><option>-fwarn-incomplete-record-updates</option></listitem>
<listitem><option>-fwarn-monomorphism-restriction</option></listitem>
<listitem><option>-fwarn-implicit-prelude</option></listitem>
</itemizedlist>
</listitem>
</varlistentry>
......@@ -913,6 +914,31 @@ ghc -c Foo.hs</screen>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-fwarn-implicit-prelude</option>:</term>
<listitem>
<indexterm><primary><option>-fwarn-implicit-prelude</option></primary></indexterm>
<indexterm><primary>implicit prelude, warning</primary></indexterm>
<para>Have the compiler warn if the Prelude is implicitly
imported. This happens unless either the Prelude module is
explicitly imported with an <literal>import ... Prelude ...</literal>
line, or this implicit import is disabled (either by
<option>-fno-implicit-prelude</option> or a
<literal>LANGUAGE NoImplicitPrelude</literal> pragma).</para>
<para>Note that no warning is given for syntax that implicitly
refers to the Prelude, even if <option>-fno-implicit-prelude</option>
would change whether it refers to the Prelude.
For example, no warning is given when
<literal>368</literal> means
<literal>Prelude.fromInteger (368::Prelude.Integer)</literal>
(where <literal>Prelude</literal> refers to the actual Prelude module,
regardless of the imports of the module being compiled).</para>
<para>This warning is off by default.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-fwarn-incomplete-patterns</option>:</term>
<listitem>
......
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