The interpreter may be started with a command line of the form
hugs [option...] [file...]
On many systems it can also be found in the system menus,
and may be started by (double) clicking on a file with a
“.hs” or
“.lhs” extension.
Hugs takes options from the command line and elsewhere
(see Section 3.1),
and then loads the Haskell Prelude module,
as well as as any modules specified on the command line.
Hugs starts with a banner like
__ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Bugs: http://hackage.haskell.org/trac/hugs || || Version: March 2005 _________________________________________ Haskell 98 mode: Restart with command line option -98 to enable extensions Type :? for help Hugs>
The prompt string Hugs> indicates that the current
module is an empty module called Hugs
(assuming no modules were specified on the command line).
At this prompt, you can type Haskell expressions to be evaluated,
and also enter commands of the form
“:”,
where cmdcmd may be abbreviated to a single letter.
expr
Evaluate a Haskell expression.
The expression cannot be broken over multiple lines.
Usually, the value is simply converted to a string
(using show) and printed:
Hugs> 1+2 3
The printing style can be changed with the
-u option
(see Section 3.1.4).
However, if expr has type
IO t
for some type t,
the resulting IO action is performed:
Hugs> print (1+2) >> putStrLn "bye" 3 bye
Usually the value produced by this action is ignored,
but this can be changed with the
+I option
(see Section 3.1.4).
If the type of expr is ambiguous,
defaulting is applied to each ambiguous
type variable v
whose constraints all have the form
C v
where C is a standard class,
and at least one of these classes is a numeric class, or is
Show, Eq or Ord.
(This is an extension of the Haskell 98 rule applied to top-level definitions
in modules, which requires a numeric class.)
It is an error if any ambiguous type variables cannot be handled in this way.
For example, consider
Hugs> reverse [] []
Here a Show constraint on the list elements arises
from Hugs's use of show to display the result,
so the type of the elements defaults to Integer,
removing the ambiguity.
:type expr
Print the type of expr,
without evaluating it.
Usually the defaulting rules (discussed above) are not applied to the type
before printing,
but this can be changed with the
+T option
(see Section 3.1.4).
:set [option...]
Set command line options.
See Section 3.1 for a list of available options.
On Win32, the new option settings are saved to the registry
under the HKEY_CURRENT_USER key,
and so persist across Hugs sessions.
To make settings persistent on other systems,
put them in the HUGSFLAGS environment variable.
If no options are given, list the available options and their current settings.
:main [argument...]
Run the main function,
with the getArgs function
returning the values specified for the arguments.
This is useful for testing programs from the interpreter.
If your Main module imports
System.Environment,
you can get the same effect using withArgs, e.g.
withArgs ["Hello", "World"] main
:quit
Exit the interpreter.