diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/ghci.xml | 77 |
1 files changed, 39 insertions, 38 deletions
diff --git a/docs/users_guide/ghci.xml b/docs/users_guide/ghci.xml index 9775c34ffe..62a9c3b34e 100644 --- a/docs/users_guide/ghci.xml +++ b/docs/users_guide/ghci.xml @@ -1657,17 +1657,19 @@ main = <coref linkend="name-binding-co"/> do { Simply use ghci to evaluate your Haskell expressions and whenever a breakpoint is hit, the debugger will enter the stage: <programlisting> -*main:Main> :break add Main 2 -Breakpoint set at (2,15) - -*main:Main> qsort [10,9..1] -Local bindings in scope: - x :: a, xs :: [a], left :: [a], right :: [a] - -qsort2.hs:2:15-46> +*main:Main> :break qsort +Breakpoint 0 activated at ../QSort.hs:(4,0)-(6,54) +*QSort> qsort [10,9,1] +Stopped at ../QSort.hs:(4,0)-(6,54) +_result :: [a] +xs :: [a] +x :: a +left :: [a] +right :: [a] +[../QSort.hs:(4,0)-(6,54)] *QSort> </programlisting> What is happening here is that GHCi has interrupted the evaluation of - <literal>qsort</literal> at the breakpoint set in line 2, as the prompt indicates. + <literal>qsort</literal> at the breakpoint, as the prompt indicates. At this point you can freely explore the contents of the bindings in scope, but with two catches. </para><para> First, take into account that due to the lazy nature of Haskell, some of @@ -1677,66 +1679,65 @@ qsort2.hs:2:15-46> GHCi has left its types parameterised by a variable! Look at the type of <literal>qsort</literal>, which is polymorphic on the type of its argument. It does not - tell us really what the types of <literal>x</literal> and <literal>xs</literal> can be. - In general, polymorphic programs deal with polymorphic values, + tell us really what the types of <literal>x</literal> + and <literal>xs</literal> can be. + In general, polymorphic functions deal with polymorphic values, and this means that some of the bindings available in a breakpoint site will be parametrically typed. </para><para> So, what can we do with a value without concrete type? Very few interesting - things. The <literal>:print</literal> command in ghci allows you to + things, not even using <literal>show</literal> on it. + The <literal>:print</literal> command in ghci allows you to explore its contents and see if it is evaluated or not. - This is useful because you cannot just type <literal>x</literal> in the - prompt and expect GHCi to return you its value. Perhaps you know for - sure that - <literal>x</literal> is of type <literal>Int</literal>, which is an instance of - <literal>Show</literal>, but GHCi does not have this information. - <literal>:print</literal> however is fine, because it does not need to know the - type to do its work. </para> + <literal>:print</literal> works here because it does not need the + type information to do its work. In fact, as we will see later, + <literal>:print</literal> can even recover the missing type information.</para> + <para> Let's go on with the debugging session of the <literal>qsort</literal> example: <example id="debuggingEx"><title>A short debugging session</title> <programlisting> qsort2.hs:2:15-46> x -This is an untyped, unevaluated computation. You can use seq to -force its evaluation and then :print to recover its type <co id="seq1"/> +<interactive>:1:0: + Ambiguous type variable `a' in the constraint: <co id="seq1"/> + `Show a' arising from a use of `print' at <interactive>:1:0 qsort2.hs:2:15-46> seq x () <co id="seq2"/> () qsort2.hs:2:15-46> x <co id="seq3"/> -This is an untyped, unevaluated computation. You can use seq to -force its evaluation and then :print to recover its type - +<interactive>:1:0: + Ambiguous type variable `a' in the constraint: + `Show a' arising from a use of `print' at <interactive>:1:0 qsort2.hs:2:15-46> :t x -x :: GHC.Base.Unknown -qsort2.hs:2:15-46> :p x <co id="seq4"/> -x - 10 +x :: a +qsort2.hs:2:15-46> :print x <co id="seq4"/> +x = 10 qsort2.hs:2:15-46> :t x <co id="seq5"/> -x :: Int +x :: Integer </programlisting> </example> <calloutlist> <callout arearefs="seq1"> - <para>GHCi reminds us that this value is untyped, and instructs us to force its evaluation </para> + <para>GHCi reminds us that <literal>x</literal> is untyped </para> </callout> <callout arearefs="seq2"> <para>This line forces the evaluation of <literal>x</literal> </para> </callout> <callout arearefs="seq3"> - <para>Even though x has been evaluated, we cannot simply use its name to see its value! - This is a bit counterintuitive, but currently in GHCi the type of a binding - cannot be a type variable <literal>a</literal>. - Thus, the binding <literal>x</literal> gets assigned the concrete type Unknown.</para> + <para>Even though x has been evaluated, + we have not updated its type yet. </para> </callout> <callout arearefs="seq4"> <para>We can explore <literal>x</literal> using the <literal>:print</literal> - command, which does find out that <literal>x</literal> is of type Int and prints - its value accordingly.</para> + command, which does find out that <literal>x</literal> is of type Int and + prints its value.</para> </callout> <callout arearefs="seq5"> - <para><literal>:print</literal> also updates the type of <literal>x</literal> with - the most concrete type information available.</para> + <para>In addition, <literal>:print</literal> also updates + its type information.</para> </callout> </calloutlist> - The example shows the standard way to proceeed with polymorphic values in a breakpoint. + + This example shows the standard way to proceeed with polymorphic values in a breakpoint. </para> </sect2> <sect2><title>Commands</title> |