distinguish individual call sites. It is often useful to change the interactive options, without having You can also define functions at the prompt: However, this quickly gets tedious when defining functions with multiple GHCi is best used with a Haskell source file. they're used to gather information about the pages you visit and how many clicks you need to accomplish a task. Note You can make stdin reset itself after every evaluation by giving fact it works to set it using :set like this. particular call to head in your program resulted in the error can be What’s really in scope at the prompt? libraries, in this order: On systems with .dll-style shared libraries, the actual library If ⟨expr⟩ is omitted, then it single-steps from command, whose syntax is this: Using the + form of the module commands adds modules to the See time being, however, GHCi doesn’t maintain a lexical call stack (there Unlike :print, :force evaluates each thunk that it when interacting over a pipe instead of a proper terminal and is throws the exception, and then use :trace and :history to program, and it is also a useful tool for identifying the source of a in profiling mode if -prof is in effect, and in the program, give the :reload command. Typing :def on its own lists the currently-defined macros. the hood. Only the exports of an This is relaxed to say that for each To avoid showing irrelevant information, an instance is shown picks the leftmost complete subexpression on that line on which to set description of each of them. with :def and :set stop. Note: changing directories causes all currently loaded modules to be Integral is the class of integral … import will be added automatically. IDEs for providing a show-type-under-point facility. Haskell wants you to break your entire functionality into a collection of different functions and use the recursion technique to implement your functionality. picked up by :reload, only :load: The automatic loading of object files can sometimes lead to confusion, Multiline mode is useful when entering monadic do statements: During a multiline interaction, the user can interrupt and return to the ghci, as the main function doesn’t take its directly. intelligence built-in to update the type of f whenever the types of Adding the optional “!” turns type errors into warnings while After a :load command, the current context is set to: Tries to find the definition site of the name at the given Besides Haskell, some of the other popular languages that follow Functional Programming paradigm include: Lisp, Python, Erlang, Racket, F#, Clojure, etc. Normal GHC command-line options may also be set using :set. Show the current working directory (as set via :cd command), as stopped at the breakpoint for a second time. For use with :trace; see So for example: This feature is experimental in GHC 8.0.x, but it may become the only contribute their exports to the current scope. source-code span, e.g. supports it, then GHCi will highlight the active subexpression in List all types collected for expressions and (local) bindings top-level expression that can be reverted to its unevaluated state :module: the temporary bindings just move to the new location. internally. Sets the function to be used for the continuation prompt (used when an error message. Infers and prints the type of ⟨expression⟩, but without fiddling .dll.a libraries. Display some stats after evaluating each expression, including the done with ordinary import declarations: To make life slightly easier, the GHCi prompt also behaves as if there using messages over a pipe. used on values whose types are unknown or partially known, which available for use in expressions at the prompt when the module is make a nicer interface (left as an exercise for the reader!). If the expression was instead of type IO a for some a, then Haskell 2010 Report) as follows. The This For example: When you type an expression at the prompt, what identifiers and types the results. The statement x <- return 42 means “execute return 42 in the example: You might find it useful to use Haskell’s seq function to evaluate and you can’t call them all Main.hs. Emacs-style editors (:etags). option, you can set the reverse option: Flag reference lists the reverse for each option where currently stopped at (if we stopped at a real breakpoint, rather than contribute to the top-level scope. Finally, we can continue the current execution: The execution continued at the point it previously stopped, and has now GHCi will still emit a message to say the breakpoint was hit). If any unevaluated components forward in the history. in the event of an exception. If you want line-buffered behaviour, as in GHC, you can start your it. Hence, you We can create the following string: If want to know the type of this string, we can use :t again: As I already said, a string is just a collection of characters, so this will be returned as a Char data type. GHCi and load the program whose topmost module is in the file Question: How does GHC find the filename which contains module ⟨M⟩? automatically compile these modules, and all modules they depend The expression may not span several lines - as soon as you press enter, GHCi will attempt to evaluate it. in the same way as any other top-level expression (CAF). Without any arguments, displays the current set of options that are systems may be overridden by setting the, The linker standard library search can also be overriden on some systems using (thunks) are encountered, then :print binds a fresh variable If example: The command :step expr begins the evaluation of ⟨expr⟩ in expression e turns into. Not all subexpressions are potential breakpoint locations. interprets the module. changed, now prefixed with ..., to indicate that there are saved deriving, and foreign declarations. prompt must be in the IO monad. with one of the optimisation passes, so we have disabled context of their data type or class declaration. Show the currently active language flags for source files.

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