X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fghci.xml;h=35aa7cd279e4cf48400447b93c1425946f8f5c92;hb=f555abffd676544cd13d022bf1eb829e63f7aebe;hp=e034021c3857fdd953090c8a0f2ee696e2fef1df;hpb=2df369da6d961f278b231aac6d21fdd25dafaf31;p=ghc-hetmet.git
diff --git a/docs/users_guide/ghci.xml b/docs/users_guide/ghci.xml
index e034021..35aa7cd 100644
--- a/docs/users_guide/ghci.xml
+++ b/docs/users_guide/ghci.xml
@@ -368,7 +368,6 @@ hello
IO monad.
Prelude> x <- return 42
-42
Prelude> print x
42
Prelude>
@@ -380,7 +379,8 @@ Prelude>
x in future statements, for example to print
it as we did above.
- GHCi will print the result of a statement if and only if:
+ If is set then
+ GHCi will print the result of a statement if and only if:
The statement is not a binding, or it is a monadic binding
@@ -393,13 +393,8 @@ Prelude>
Show
- The automatic printing of binding results can be supressed with
- (this does not
- supress printing the result of non-binding statements).
- .
- You might want to do this to prevent the result of binding
- statements from being fully evaluated by the act of printing
- them, for example.
+ .
+ Of course, you can also bind normal non-IO expressions
using the let-statement:
@@ -424,6 +419,45 @@ Prelude>
Note that let bindings do not automatically
print the value bound, unlike monadic bindings.
+ Hint: you can also use let-statements
+ to define functions at the prompt:
+
+Prelude> let add a b = a + b
+Prelude> add 1 2
+3
+Prelude>
+
+ However, this quickly gets tedious when defining functions
+ with multiple clauses, or groups of mutually recursive functions,
+ because the complete definition has to be given on a single line,
+ using explicit braces and semicolons instead of layout:
+
+Prelude> let { f op n [] = n ; f op n (h:t) = h `op` f op n t }
+Prelude> f (+) 0 [1..3]
+6
+Prelude>
+
+ To alleviate this issue, GHCi commands can be split over
+ multiple lines, by wrapping them in :{ and
+ :} (each on a single line of its own):
+
+Prelude> :{
+Prelude| let { g op n [] = n
+Prelude| ; g op n (h:t) = h `op` g op n t
+Prelude| }
+Prelude| :}
+Prelude> g (*) 1 [1..3]
+6
+
+ Such multiline commands can be used with any GHCi command,
+ and the lines between :{ and
+ :} are simply merged into a single line for
+ interpretation. That implies that each such group must form a single
+ valid command when merged, and that no layout rule is used.
+ The main purpose of multiline commands is not to replace module
+ loading but to make definitions in .ghci-files (see ) more readable and maintainable.
+
Any exceptions raised during the evaluation or execution
of the statement are caught and printed by the GHCi command line
interface (for more information on exceptions, see the module
@@ -577,7 +611,7 @@ Prelude IO>
- The :main command
+ The :main and :run commands
When a program is compiled and executed, it can use the
@@ -602,6 +636,37 @@ Prelude> :main foo bar
["foo","bar"]
+
+ We can also quote arguments which contains characters like
+ spaces, and they are treated like Haskell strings, or we can
+ just use Haskell list syntax:
+
+
+
+Prelude> :main foo "bar baz"
+["foo","bar baz"]
+Prelude> :main ["foo", "bar baz"]
+["foo","bar baz"]
+
+
+
+ Finally, other functions can be called, either with the
+ -main-is flag or the :run
+ command:
+
+
+
+Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
+Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print
+Prelude> :set -main-is foo
+Prelude> :main foo "bar baz"
+foo
+["foo","bar baz"]
+Prelude> :run bar ["foo", "bar baz"]
+bar
+["foo","bar baz"]
+
+
@@ -780,10 +845,12 @@ def = toEnum 0
GHCi contains a simple imperative-style debugger in which you can
stop a running computation in order to examine the values of
variables. The debugger is integrated into GHCi, and is turned on by
- default: no flags are required to enable the debugging facilities. There
- is one major restriction: breakpoints and single-stepping are only
- available in interpreted modules; compiled code is
- invisible to the debugger.
+ default: no flags are required to enable the debugging
+ facilities. There is one major restriction: breakpoints and
+ single-stepping are only available in interpreted modules;
+ compiled code is invisible to the debuggerNote that packages
+ only contain compiled code, so debugging a package requires
+ finding its source and loading that directly..
The debugger provides the following:
@@ -818,9 +885,12 @@ def = toEnum 0
There is currently no support for obtaining a “stack
- trace”, but the tracing and history features provide a useful
- second-best, which will often be enough to establish the context of an
- error.
+ trace”, but the tracing and history features provide a
+ useful second-best, which will often be enough to establish the
+ context of an error. For instance, it is possible to break
+ automatically when an exception is thrown, even if it is thrown
+ from within compiled code (see ).
Breakpoints and inspecting variables
@@ -929,6 +999,7 @@ right :: [a]
left:
+[qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
[qsort.hs:2:15-46] *Main> :print left
left = (_t1::[a])
@@ -948,6 +1019,13 @@ left = (_t1::[a])
underscore, in this case
_t1.
+ The flag -fprint-evld-with-show instructs
+ :print to reuse
+ available Show instances when possible. This happens
+ only when the contents of the variable being inspected
+ are completely evaluated.
+
+
If we aren't concerned about preserving the evaluatedness of a
variable, we can use :force instead of
:print. The :force command
@@ -1017,6 +1095,7 @@ right :: [a]
The execution continued at the point it previously stopped, and has
now stopped at the breakpoint for a second time.
+
Setting breakpoints
@@ -1106,10 +1185,14 @@ right :: [a]
Single-stepping is a great way to visualise the execution of your
program, and it is also a useful tool for identifying the source of a
- bug. The concept is simple: single-stepping enables all the
- breakpoints in the program and executes until the next breakpoint is
- reached, at which point you can single-step again, or continue
- normally. For example:
+ bug. GHCi offers two variants of stepping. Use
+ :step to enable all the
+ breakpoints in the program, and execute until the next breakpoint is
+ reached. Use :steplocal to limit the set
+ of enabled breakpoints to those in the current top level function.
+ Similarly, use :stepmodule to single step only on
+ breakpoints contained in the current module.
+ For example:
*Main> :step main
@@ -1118,10 +1201,11 @@ _result :: IO ()
The command :step
- expr begins the evaluation of
+ expr begins the evaluation of
expr in single-stepping mode. If
- expr is ommitted, then it single-steps from
- the current breakpoint.
+ expr is omitted, then it single-steps from
+ the current breakpoint. :stepover
+ works similarly.
The :list command is particularly useful when
single-stepping, to see where you currently are:
@@ -1330,9 +1414,13 @@ a :: a
:trace and :history to establish
the context. However, head is in a library and
we can't set a breakpoint on it directly. For this reason, GHCi
- provides the flag -fbreak-on-exception which causes
- the evaluator to stop when an exception is thrown, just as it does when
- a breakpoint is hit. This is only really useful in conjunction with
+ provides the flags -fbreak-on-exception which causes
+ the evaluator to stop when an exception is thrown, and
+ -fbreak-on-error, which works similarly but stops only on
+ uncaught exceptions. When stopping at an exception, GHCi will act
+ just as it does when a breakpoint is hit, with the deviation that it
+ will not show you any source code location. Due to this, these
+ commands are only really useful in conjunction with
:trace, in order to log the steps leading up to the
exception. For example:
@@ -1385,7 +1473,7 @@ as = 'b' : 'c' : (_t1::[Char])
import Prelude hiding (map)
-map :: (a->b) -> a -> b
+map :: (a->b) -> [a] -> [b]
map f [] = []
map f (x:xs) = f x : map f xs
@@ -1662,22 +1750,58 @@ $ ghci -lm
- :browse*module ...
+ :browse!*module ...
:browseDisplays the identifiers defined by the module
module, which must be either
- loaded into GHCi or be a member of a package. If the
- * symbol is placed before the module
- name, then all the identifiers defined
- in module are shown; otherwise
- the list is limited to the exports of
+ loaded into GHCi or be a member of a package. If
+ module is omitted, the most
+ recently-loaded module is used.
+
+ If the * symbol is placed before
+ the module name, then all the
+ identifiers in scope in module are
+ shown; otherwise the list is limited to the exports of
module. The
*-form is only available for modules
which are interpreted; for compiled modules (including
modules from packages) only the non-*
- form of :browse is available.
+ form of :browse is available.
+ If the ! symbol is appended to the
+ command, data constructors and class methods will be
+ listed individually, otherwise, they will only be listed
+ in the context of their data type or class declaration.
+ The !-form also annotates the listing
+ with comments giving possible imports for each group of
+ entries.
+
+Prelude> :browse! Data.Maybe
+-- not currently imported
+Data.Maybe.catMaybes :: [Maybe a] -> [a]
+Data.Maybe.fromJust :: Maybe a -> a
+Data.Maybe.fromMaybe :: a -> Maybe a -> a
+Data.Maybe.isJust :: Maybe a -> Bool
+Data.Maybe.isNothing :: Maybe a -> Bool
+Data.Maybe.listToMaybe :: [a] -> Maybe a
+Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
+Data.Maybe.maybeToList :: Maybe a -> [a]
+-- imported via Prelude
+Just :: a -> Maybe a
+data Maybe a = Nothing | Just a
+Nothing :: Maybe a
+maybe :: b -> (a -> b) -> Maybe a -> b
+
+
+ This output shows that, in the context of the current session, in the scope
+ of Prelude, the first group of items from
+ Data.Maybe have not been imported (but are available in
+ fully qualified form in the GHCi session - see ), whereas the second group of items have been
+ imported via Prelude and are therefore available either
+ unqualified, or with a Prelude. qualifier.
+
@@ -1739,7 +1863,7 @@ $ ghci -lm
Generates a “tags” file for Vi-style editors
(:ctags) or
Emacs-style editors (:etags). If
- no filename is specified, the defaulit tags or
+ no filename is specified, the default tags or
TAGS is
used, respectively. Tags for all the functions, constructors and
types in the currently loaded modules are created. All modules must
@@ -1750,26 +1874,27 @@ $ ghci -lm
- :defnameexpr
+ :def!nameexpr:def
- The command :def
- name
- expr defines a new GHCi command
- :name,
- implemented by the Haskell expression
- expr, which must have type
- String -> IO String. When
- :name
- args is typed at the
- prompt, GHCi will run the expression
- (name
- args), take the
- resulting String, and feed it back into
- GHCi as a new sequence of commands. Separate commands in
- the result must be separated by
- ‘\n’.
+ :def is used to define new
+ commands, or macros, in GHCi. The command
+ :defname
+ expr defines a new GHCi command
+ :name,
+ implemented by the Haskell expression
+ expr, which must have type
+ String -> IO String. When
+ :name
+ args is typed at the
+ prompt, GHCi will run the expression
+ (name
+ args), take the
+ resulting String, and feed it back into
+ GHCi as a new sequence of commands. Separate commands in
+ the result must be separated by
+ ‘\n’.That's all a little confusing, so here's a few
examples. To start with, here's a new GHCi command which
@@ -1813,6 +1938,12 @@ Prelude> :. cmds.ghci
:., by analogy with the
‘.’ Unix shell command that
does the same thing.
+
+ Typing :def on its own lists the
+ currently-defined macros. Attempting to redefine an
+ existing command name results in an error unless the
+ :def! form is used, in which case the old
+ command with that name is silently overwritten.
@@ -1896,6 +2027,17 @@ Prelude> :. cmds.ghci
+
+ :
+ :
+
+
+ Repeat the previous command.
+
+
+
+
+
:history [num]:history
@@ -1923,6 +2065,12 @@ Prelude> :. cmds.ghci
will be printed. If name has
been loaded from a source file, then GHCi will also display
the location of its definition in the source.
+ For types and classes, GHCi also summarises instances that
+ mention them. To avoid showing irrelevant information, an instance
+ is shown only if (a) its head mentions name,
+ and (b) all the other things mentioned in the instance
+ are in scope (either qualified or otherwise) as a result of
+ a :load or :module commands.
@@ -2007,6 +2155,37 @@ Prelude> :main foo bar
["foo","bar"]
+
+ We can also quote arguments which contains characters like
+ spaces, and they are treated like Haskell strings, or we can
+ just use Haskell list syntax:
+
+
+
+Prelude> :main foo "bar baz"
+["foo","bar baz"]
+Prelude> :main ["foo", "bar baz"]
+["foo","bar baz"]
+
+
+
+ Finally, other functions can be called, either with the
+ -main-is flag or the :run
+ command:
+
+
+
+Prelude> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
+Prelude> let bar = putStrLn "bar" >> System.Environment.getArgs >>= print
+Prelude> :set -main-is foo
+Prelude> :main foo "bar baz"
+foo
+["foo","bar baz"]
+Prelude> :run bar ["foo", "bar baz"]
+bar
+["foo","bar baz"]
+
+
@@ -2081,10 +2260,11 @@ Prelude> :main foo bar
:set
- Sets various options. See
- for a list of available options. The
- :set command by itself shows which
- options are currently set.
+ Sets various options. See for a list of
+ available options and for a
+ list of GHCi-specific flags. The :set command by
+ itself shows which options are currently set. It also lists the current
+ dynamic flag settings, with GHCi-specific flags listed separately.
@@ -2207,6 +2387,28 @@ Prelude> :main foo bar
+ :show packages
+ :show packages
+
+
+ Show the currently active package flags, as well as the list of
+ packages currently loaded.
+
+
+
+
+
+ :show languages
+ :show languages
+
+
+ Show the currently active language flags.
+
+
+
+
+
+ :show [args|prog|prompt|editor|stop]:show
@@ -2424,18 +2626,35 @@ Prelude> :set -fno-glasgow-exts
startupfiles, GHCi
- When it starts, GHCi always reads and executes commands from
- $HOME/.ghci, followed by
- ./.ghci.
-
- The .ghci in your home directory is
- most useful for turning on favourite options (eg. :set
- +s), and defining useful macros. Placing a
- .ghci file in a directory with a Haskell
- project is a useful way to set certain project-wide options so you
- don't have to type them everytime you start GHCi: eg. if your
- project uses GHC extensions and CPP, and has source files in three
- subdirectories A B and C, you might put the following lines in
+ When it starts, unless the -ignore-dot-ghci
+ flag is given, GHCi reads and executes commands from the following
+ files, in this order, if they exist:
+
+
+
+ ./.ghci
+
+
+ appdata/ghc/ghci.conf,
+ where appdata depends on your system,
+ but is usually something like C:/Documents and Settings/user/Application Data
+
+
+ On Unix: $HOME/.ghc/ghci.conf
+
+
+ $HOME/.ghci
+
+
+
+ The ghci.conf file is most useful for
+ turning on favourite options (eg. :set +s), and
+ defining useful macros. Placing a .ghci file
+ in a directory with a Haskell project is a useful way to set
+ certain project-wide options so you don't have to type them
+ everytime you start GHCi: eg. if your project uses GHC extensions
+ and CPP, and has source files in three subdirectories A, B and C,
+ you might put the following lines in
.ghci:
@@ -2449,7 +2668,7 @@ Prelude> :set -fno-glasgow-exts
until the next :load, though.)Two command-line options control whether the
- .ghci files are read:
+ startup files files are read:
@@ -2458,8 +2677,8 @@ Prelude> :set -fno-glasgow-exts
- Don't read either ./.ghci or
- $HOME/.ghci when starting up.
+ Don't read either ./.ghci or the
+ other startup files when starting up.
@@ -2468,8 +2687,8 @@ Prelude> :set -fno-glasgow-exts
- Read .ghci and
- $HOME/.ghci. This is normally the
+ Read ./.ghci and the other
+ startup files (see above). This is normally the
default, but the option may
be used to override a previous
option.
@@ -2583,7 +2802,19 @@ Prelude> :set -fno-glasgow-exts
I can't use Control-C to interrupt computations in
GHCi on Windows.
- See
+ See .
+
+
+
+
+ The default buffering mode is different in GHCi to GHC.
+
+
+ In GHC, the stdout handle is line-buffered by default.
+ However, in GHCi we turn off the buffering on stdout,
+ because this is normally what you want in an interpreter:
+ output appears as it is generated.
+