X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=docs%2Fusers_guide%2Fghci.xml;h=54ce6b7c74ea8d558c4abd48c21560fe49e90775;hp=61aa5132f96c0ec009aa9c8b59c078647854446e;hb=45b04f650e6985c524301112a8381a1aff808fc6;hpb=a4a6817fc78562170d00852595cf6af3fa38d209
diff --git a/docs/users_guide/ghci.xml b/docs/users_guide/ghci.xml
index 61aa513..54ce6b7 100644
--- a/docs/users_guide/ghci.xml
+++ b/docs/users_guide/ghci.xml
@@ -1831,22 +1831,90 @@ x :: Int
- Limitations
-
-
-
- Implicit parameters (see ) are only available
- at the scope of a breakpoint if there is a explicit type signature.
-
-
-
-
- Modules compiled by GHCi under the -fdebugging
- flag will perform slower: the debugging mode introduces some overhead.
- Modules compiled to object code by ghc are not affected.
-
-
-
+ Debugging Higher-Order functions
+ 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 map function:
+
+import Prelude hiding (map)
+
+map :: (a->b) -> a -> b
+map f [] = []
+map f (x:xs) = f x : map f xs
+
+ We set a breakpoint on map, and call it.
+
+*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]
+
+ GHCi tells us that, among other bindings, f 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 x, and its result type is the
+ same as the type of _result.
+ The debugger has some intelligence built-in to update the type of
+ f whenever the types of x or
+ _result are reconstructed. So what we do in this scenario is
+ force x a bit, in order to recover both its type
+ and the argument part of f.
+
+*Main> seq x ()
+*Main> :print x
+x = 1
+
+ We can check now that as expected, the type of x
+ has been reconstructed, and with it the
+ type of f has been too:
+
+*Main> :t x
+x :: Integer
+*Main> :t f
+f :: Integer -> b
+
+ From here, we can apply f to any argument of type Integer and observe the
+ results.
+ let b = f 10
+*Main> :t b
+b :: b
+*Main> b
+:1:0:
+ Ambiguous type variable `b' in the constraint:
+ `Show b' arising from a use of `print' at :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]
+ ]]>
+ In the first application of f, we had to do
+ some more type reconstruction
+ in order to recover the result type of f.
+ But after that, we are free to use
+ f normally.
Tips
@@ -1865,8 +1933,8 @@ x :: Int
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'
-
+
+ Limitations
+
+
+
+ Implicit parameters (see ) are only available
+ at the scope of a breakpoint if there is a explicit type signature.
+
+
+
+
+ Modules compiled by GHCi under the -fdebugging
+ flag will perform slower: the debugging mode introduces some overhead.
+ Modules compiled to object code by ghc are not affected.
+
+
+
+
+
The .ghci file
.ghcifile