Haskell/Using GHCi effectively



GHCi offers several ways to make you work faster. This section will describe them and explain what they are for.

User interface

Tab completion

Whenever you are typing anything into the GHCi, you can hit Tab and you will be presented with a list of all possible names that start with what you've written so far. And if there is only one way how to continue, the string will be auto-completed. For example fol<Tab> will append letter d, another Tab will list four functions: fold foldl1 foldr and foldr1. Only functions and other defined objects (e.g. Data types) from modules that are currently imported are offered.

Tab completion works also when you are loading a file with your program into GHCi. After typing :l fi<Tab>, you will be presented with all files that start with "fi" and are present in the current directory (the one you were in when you launched GHCi).

": commands"

On GHCi command line, commands for the interpreter start with the character ":" (colon).

•  :help -- prints a list of all available commands.
•  :load myfile.hs -- loads a file (here, "myfile.hs") into GHCi.
•  :reload -- reloads the file that has been loaded last time, so changes are visible from GHCi.
•  :type <Haskell expression> -- prints the type of a given expression, e.g. a function that has been defined previously.
•  :module +Data.List -- loads a module (in this example, Data.List). You can also unload a module with :module -Data.List.
•  :browse Data.List -- gives the type signatures for all functions available from a module.

Many of these commands can be abbreviated - :l for :load, :r for :reload, :h for :help, :t for :type and :m for module.

Timing Functions in GHCi

GHCi provides a basic way to measure how much time a function takes to run, which can be useful for people who quickly want to find out, for example, which version of a function runs faster.

1. Type :set +s into the ghci command line.
2. run the function(s) you are testing. The time the function took to run will be displayed after GHCi outputs the results of the function.

Multi-line Input

If you are trying to define a function that takes up multiple lines, or if you want to type a do block into ghci, there is an easy way to do this:

1. Begin a new line with :{
2. Type in your code. Press enter when you need a new line.
3. Type :} to end the multiline input.

For example:

   *Main> :{
   *Main| let askname = do
   *Main|               putStrLn "What is your name?"
   *Main|               name <- getLine
   *Main|               putStrLn $ "Hello " ++ name
   *Main| :}
   *Main>

In addition, line breaks in ghci commands can be separated by ;, like this:

   *Main> let askname1 = do ; putStrLn "what is your name?" ; name <- getLine ; putStrLn $ "Hello " ++ name