diff options
| -rw-r--r-- | docs/users_guide/ghci.xml | 14 |
1 files changed, 10 insertions, 4 deletions
diff --git a/docs/users_guide/ghci.xml b/docs/users_guide/ghci.xml index 62a9c3b34e..5d697e6b2b 100644 --- a/docs/users_guide/ghci.xml +++ b/docs/users_guide/ghci.xml @@ -1826,14 +1826,15 @@ x :: Integer </variablelist></para> </sect2> <sect2><title>Debugging Higher-Order functions</title> + <para> 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. - + </para><para> 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: + We show the process by means of an example. + To keep things simple, we will use the well known <literal>map</literal> function: <programlisting> import Prelude hiding (map) @@ -1841,6 +1842,7 @@ map :: (a->b) -> a -> b map f [] = [] map f (x:xs) = f x : map f xs </programlisting> + </para><para> We set a breakpoint on <literal>map</literal>, and call it. <programlisting> *Main> :break map @@ -1854,10 +1856,11 @@ 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 + and thus it is not possible to apply it 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>. + </para><para> 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 @@ -1868,6 +1871,7 @@ xs :: [a] *Main> :print x x = 1 </programlisting> + </para><para> 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: @@ -1877,6 +1881,7 @@ x :: Integer *Main> :t f f :: Integer -> b </programlisting> + </para><para> From here, we can apply f to any argument of type Integer and observe the results. <programlisting><![CDATA[ @@ -1909,6 +1914,7 @@ Just 20 in order to recover the result type of <literal>f</literal>. But after that, we are free to use <literal>f</literal> normally. + </para> </sect2> <sect2><title>Tips</title> <variablelist> |