Commit 45b04f65 authored by mnislaih's avatar mnislaih

New section on debugging lambdas in the ghci user guide

parent a4a6817f
......@@ -1831,22 +1831,90 @@ x :: Int
</varlistentry>
</variablelist></para>
</sect2>
<sect2><title>Limitations</title>
<para>
<itemizedlist>
<listitem><para>
Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
at the scope of a breakpoint if there is a explicit type signature.
</para></listitem>
</itemizedlist>
<itemizedlist>
<listitem><para>
Modules compiled by GHCi under the <literal>-fdebugging
</literal> flag will perform slower: the debugging mode introduces some overhead.
Modules compiled to object code by ghc are not affected.
</para></listitem>
</itemizedlist>
</para>
<sect2><title>Debugging Higher-Order functions</title>
It is possible to use the debugger to examine lambdas.
When we are at a breakpoint and a lambda is in scope, the debugger cannot show
you the source code that constitutes it; however, it is possible to get some
information by applying it to some arguments and observing the result.
The process is slightly complicated when the binding is polymorphic.
We will use a example to show the process.
To keep it simple, we will use the well known <literal>map</literal> function:
<programlisting>
import Prelude hiding (map)
map :: (a->b) -> a -> b
map f [] = []
map f (x:xs) = f x : map f xs
</programlisting>
We set a breakpoint on <literal>map</literal>, and call it.
<programlisting>
*Main> :break map
Breakpoint 0 activated at map.hs:(4,0)-(5,12)
*Main> map Just [1..5]
Stopped at map.hs:(4,0)-(5,12)
_result :: [b]
x :: a
f :: a -> b
xs :: [a]
</programlisting>
GHCi tells us that, among other bindings, <literal>f</literal> is in scope.
However, its type is not fully known yet,
and thus it is not possible to apply it yet to any
arguments. Nevertheless, observe that the type of its first argument is the
same as the type of <literal>x</literal>, and its result type is the
same as the type of <literal>_result</literal>.
The debugger has some intelligence built-in to update the type of
<literal>f</literal> whenever the types of <literal>x</literal> or
<literal>_result</literal> are reconstructed. So what we do in this scenario is
force <literal>x</literal> a bit, in order to recover both its type
and the argument part of <literal>f</literal>.
<programlisting>
*Main> seq x ()
*Main> :print x
x = 1
</programlisting>
We can check now that as expected, the type of <literal>x</literal>
has been reconstructed, and with it the
type of <literal>f</literal> has been too:
<programlisting>
*Main> :t x
x :: Integer
*Main> :t f
f :: Integer -> b
</programlisting>
From here, we can apply f to any argument of type Integer and observe the
results.
<programlisting><![CDATA[
*Main> let b = f 10
*Main> :t b
b :: b
*Main> b
<interactive>:1:0:
Ambiguous type variable `b' in the constraint:
`Show b' arising from a use of `print' at <interactive>:1:0
*Main> :p b
b = (_t2::a)
*Main> seq b ()
()
*Main> :t b
b :: a
*Main> :p b
b = Just 10
*Main> :t b
b :: Maybe Integer
*Main> :t f
f :: Integer -> Maybe Integer
*Main> f 20
Just 20
*Main> map f [1..5]
[Just 1, Just 2, Just 3, Just 4, Just 5]
]]></programlisting>
In the first application of <literal>f</literal>, we had to do
some more type reconstruction
in order to recover the result type of <literal>f</literal>.
But after that, we are free to use
<literal>f</literal> normally.
</sect2>
<sect2><title>Tips</title>
<variablelist>
......@@ -1865,8 +1933,8 @@ x :: Int
<listitem><para><programlisting>
type MyLongType a = [Maybe [Maybe a]]
main:Main> :m +GHC.Exts
main:Main> main
*Main> :m +GHC.Exts
*Main> main
Local bindings in scope:
x :: a
Main.hs:15> let x' = unsafeCoerce x :: MyLongType Bool
......@@ -1885,7 +1953,25 @@ Main.hs:15> x'
</para></listitem>
</varlistentry>
</variablelist>
</sect2></sect1>
</sect2>
<sect2><title>Limitations</title>
<para>
<itemizedlist>
<listitem><para>
Implicit parameters (see <xref linkend="implicit-parameters"/>) are only available
at the scope of a breakpoint if there is a explicit type signature.
</para></listitem>
</itemizedlist>
<itemizedlist>
<listitem><para>
Modules compiled by GHCi under the <literal>-fdebugging
</literal> flag will perform slower: the debugging mode introduces some overhead.
Modules compiled to object code by ghc are not affected.
</para></listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="ghci-dot-files">
<title>The <filename>.ghci</filename> file</title>
<indexterm><primary><filename>.ghci</filename></primary><secondary>file</secondary>
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment