I'm trying to process arrow key ANSI escape sequences i.e.
up - "\033[A"
down - "\033[B"
left - "\033[D"
right - "\033[C"
in my programme so when I press the up/down/left/right arrow key, it won't have to look like this:
% stack runghc test.hs
Input a name?
^[[A^[[B^[[C^[[D^
on my stdin, but rather I would like those keys to be suppressed or even better,
for them to actually work(i.e move the cursor left/right). My code is as follows:
main = do putStrLn "Input a name?"
name <- getLine
putStrLn $ ("His name is " ++ name)
Any help would be appreciated. Thanks in advance.
The easiest way to get readline-like functionality is to just use readline. For most simple use cases, rlwrap is good enough, as described in One REPL to bind them all?. If you need to do fancier integrations, you can use the readline package.
I had trouble installing the readline library due to some errors and decided to use the haskeline library, which is a more portable pure-haskell replacement for it.
Using its syntax and modifying the earlier code, I got:
main :: IO ()
main = do putStrLn "Input a name?"
runInputT defaultSettings insertion
where
insertion :: InputT IO ()
insertion = do
minput <- getInputLine ""
case minput of
Nothing -> return ()
Just input -> do outputStrLn $ "His name is " ++ input
Which solves the problem as I am now able to move my cursor with the arrow keys freely without having to see any trailing ANSI escape sequences as shown below:
When I run this code:
test1 :: Int -> String
test1 x = do
if x == 1
then "Hello"
I get the following error:
test-if.hs:4:21: error:
parse error (possibly incorrect indentation or mismatched brackets)
I am not sure why this is as I am not using any brackets and I am using 4 spaces as my tabs. Adding brackets doesn't seem to help. What could be the issue?
Thanks
Your if needs an else (what do you want the value to be when x isn't 1?).
Furthermore do notation is used when working with monads and doesn't make sense in this function.
I'm learning some Haskell and I came across this small program
reverseLines :: String -> String
reverseLines input =
unlines (map reverse (lines input))
main :: IO ()
main = interact reverseLines
This program will keep asking the user for more input and reverse the input and print it on the screen.
Most of this is straight forward but one thing I can't wrap my head around is why does this function keeps running and ask the user for more input whereas if I just replace the reverseLines function with a function the simply returns some string it will not happen.
This program will stop after one execution:
foo input = "Stops"
main :: IO ()
main = interact foo
Why?
If you look at the source of interact you see this:
interact f = do s <- getContents
putStr (f s)
see the getContents? This is where the magic starts - it will read everything till EOF
Now in Haskell this is lazy-IO which can be bad but here is almost magical - see the string is read lazily and passed to your reverseLines - this one of course will only generate output as soon as it saw \n characters (the lines) and so it seems your program is some kind of REPL.
In the second one you don't consume any of the lazy-string at all so it stops ASAP
As I wrote in the comments you can play with this by either passing content into the program using a file (or echo) and pipes on the terminal:
echo "Hello World\nBye Bye" | runhaskell LazyIO.hs
or using CTRL-D to pass in the EOF yourself.
To get a feeling for it I would play with the functions more - what happens if you use something that needs to see the complete input first (try reverse without the maps)? What happens with words instead of lines, ...?
Have fun!
I am trying to use hint package from hackage to create a simple environment where user can issue lines of code for evaluation (like in ghci). I expect some of the input lines to be erroneous (eval would end the session with an error). How can I create a robust session that ignores erroneous input (or better: it reports an error but can accept other input) and keeps the previously consistent state?
Also, I would like to use it in do style, i.e. let a = 3 as standalone input line makes sense.
To clarify things: I have no problem with a single eval. What I would like to do, is allow continuing evaluation even after some step failed. Also I would like to incrementally extend a monadic chain (as you do in ghci I guess).
In other words: I want something like this, except that I get to evaluate 3 and don't stop at undefined with the error.
runInterpreter $ setImports [ "Prelude" ] >> eval "undefined" >> eval "3"
More specifically I would like something like this to be possible:
runInterpreter $ setImports ... >> eval' "let a = (1, 2)" -- modifying context
>> typeOf "b" -- error but not breaking the chain
>> typeOf "a" -- (Num a, Num b) => (a, b)
I don't expect it to work this straightforwardly, this is just to show the idea. I basically would like to build up some context (as you do in ghci) and every addition to the context would modify it only if there is no failure, failures could be logged or explicitly retrieved after each attempt to modify the context.
You didn't show any code so I don't know the problem. The most straight-forward way I use hint handles errors fine:
import Language.Haskell.Interpreter
let doEval s = runInterpreter $ setImports ["Prelude"] >> eval s
has resulted in fine output for me...
Prelude Language.Haskell.Interpreter> doEval "1 + 2"
Right "3"
Prelude Language.Haskell.Interpreter> doEval "1 + 'c'"
ghc: panic! (the 'impossible' happened)
(GHC version 7.10.2 for x86_64-apple-darwin):
nameModule doEval_a43r
... Except that now the impossible happens... that's a bug. Notice you are supposed to get Left someError in cases like these:
data InterpreterError
= UnknownError String
| WontCompile [GhcError]
| NotAllowed String
| GhcException String
-- Defined in ‘hint-0.4.2.3:Hint.Base’
Have you looked through the ghchq bug list and/or submitted an issue?
EDIT:
And the correct functionality is back, at least as of GHC 7.10.3 x64 on OS X with hint version 0.4.2.3. In other words, it appears the bug went away from 7.10.2 to 7.10.3
The output is:
Left (WontCompile [GhcError {errMsg = ":3:3:\n No instance for (Num Char) arising from a use of \8216+\8217\n In the expression: 1 + 'c'\n In an equation for \8216e_11\8217: e_11 = 1 + 'c'\n In the first argument of \8216show_M439719814875238119360034\8217, namely\n \8216(let e_11 = 1 + 'c' in e_11)\8217"}])
Though executing the doEval line twice in GHCi does cause a panic, things seem to work once in the interpreter and properly regardless when compiled.
For instance:
let x = 1 in putStrLn [dump|x, x+1|]
would print something like
x=1, (x+1)=2
And even if there isn't anything like this currently, would it be possible to write something similar?
TL;DR There is this package which contains a complete solution.
install it via cabal install dump
and/or
read the source code
Example usage:
{-# LANGUAGE QuasiQuotes #-}
import Debug.Dump
main = print [d|a, a+1, map (+a) [1..3]|]
where a = 2
which prints:
(a) = 2 (a+1) = 3 (map (+a) [1..3]) = [3,4,5]
by turnint this String
"a, a+1, map (+a) [1..3]"
into this expression
( "(a) = " ++ show (a) ++ "\t " ++
"(a+1) = " ++ show (a + 1) ++ "\t " ++
"(map (+a) [1..3]) = " ++ show (map (+ a) [1 .. 3])
)
Background
Basically, I found that there are two ways to solve this problem:
Exp -> String The bottleneck here is pretty-printing haskell source code from Exp and cumbersome syntax upon usage.
String -> Exp The bottleneck here is parsing haskell to Exp.
Exp -> String
I started out with what #kqr put together, and tried to write a parser to turn this
["GHC.Classes.not x_1627412787 = False","x_1627412787 = True","x_1627412787 GHC.Classes.== GHC.Types.True = True"]
into this
["not x = False","x = True","x == True = True"]
But after trying for a day, my parsec-debugging-skills have proven insufficient to date, so instead I went with a simple regular expression:
simplify :: String -> String
simplify s = subRegex (mkRegex "_[0-9]+|([a-zA-Z]+\\.)+") s ""
For most cases, the output is greatly improved.
However, I suspect this to likely mistakenly remove things it shouldn't.
For example:
$(dump [|(elem 'a' "a.b.c", True)|])
Would likely return:
["elem 'a' \"c\" = True","True = True"]
But this could be solved with proper parsing.
Here is the version that works with the regex-aided simplification: https://github.com/Wizek/kqr-stackoverflow/blob/master/Th.hs
Here is a list of downsides / unresolved issues I've found with the Exp -> String solution:
As far as I know, not using Quasi Quotation requires cumbersome syntax upon usage, like: $(d [|(a, b)|]) -- as opposed to the more succinct [d|a, b|]. If you know a way to simplify this, please do tell!
As far as I know, [||] needs to contain fully valid Haskell, which pretty much necessitates the use of a tuple inside further exacerbating the syntactic situation. There is some upside to this too, however: at least we don't need to scratch our had where to split the expressions since GHC does that for us.
For some reason, the tuple only seemed to accept Booleans. Weird, I suspect this should be possible to fix somehow.
Pretty pretty-printing Exp is not very straight-forward. A more complete solution does require a parser after all.
Printing an AST scrubs the original formatting for a more uniform looks. I hoped to preserve the expressions letter-by-letter in the output.
The deal-breaker was the syntactic over-head. I knew I could get to a simpler solution like [d|a, a+1|] because I have seen that API provided in other packages. I was trying to remember where I saw that syntax. What is the name...?
String -> Exp
Quasi Quotation is the name, I remember!
I remembered seeing packages with heredocs and interpolated strings, like:
string = [qq|The quick {"brown"} $f {"jumps " ++ o} the $num ...|]
where f = "fox"; o = "over"; num = 3
Which, as far as I knew, during compile-time, turns into
string = "The quick " ++ "brown" ++ " " ++ $f ++ "jumps " ++ o ++ " the" ++ show num ++ " ..."
where f = "fox"; o = "over"; num = 3
And I thought to myself: if they can do it, I should be able to do it too!
A bit of digging in their source code revealed the QuasiQuoter type.
data QuasiQuoter = QuasiQuoter {quoteExp :: String -> Q Exp}
Bingo, this is what I want! Give me the source code as string! Ideally, I wouldn't mind returning string either, but maybe this will work. At this point I still know quite little about Q Exp.
After all, in theory, I would just need to split the string on commas, map over it, duplicate the elements so that first part stays string and the second part becomes Haskell source code, which is passed to show.
Turning this:
[d|a+1|]
into this:
"a+1" ++ " = " ++ show (a+1)
Sounds easy, right?
Well, it turns out that even though GHC most obviously is capable to parse haskell source code, it doesn't expose that function. Or not in any way we know of.
I find it strange that we need a third-party package (which thankfully there is at least one called haskell-src-meta) to parse haskell source code for meta programming. Looks to me such an obvious duplication of logic, and potential source of mismatch -- resulting in bugs.
Reluctantly, I started looking into it. After all, if it is good enough for the interpolated-string folks (those packaged did rely on haskell-src-meta) then maybe it will work okay for me too for the time being.
And alas, it does contain the desired function:
Language.Haskell.Meta.Parse.parseExp :: String -> Either String Exp
Language.Haskell.Meta.Parse
From this point it was rather straightforward, except for splitting on commas.
Right now, I do a very simple split on all commas, but that doesn't account for this case:
[d|(1, 2), 3|]
Which fails unfortunatelly. To handle this, I begun writing a parsec parser (again) which turned out to be more difficult than anticipated (again). At this point, I am open to suggestions. Maybe you know of a simple parser that handles the different edge-cases? If so, tell me in a comment, please! I plan on resolving this issue with or without parsec.
But for the most use-cases: it works.
Update at 2015-06-20
Version 0.2.1 and later correctly parses expressions even if they contain commas inside them. Meaning [d|(1, 2), 3|] and similar expressions are now supported.
You can
install it via cabal install dump
and/or
read the source code
Conclusion
During the last week I've learnt quite a bit of Template Haskell and QuasiQuotation, cabal sandboxes, publishing a package to hackage, building haddock docs and publishing them, and some things about Haskell too.
It's been fun.
And perhaps most importantly, I now am able to use this tool for debugging and development, the absence of which has been bugging me for some time. Peace at last.
Thank you #kqr, your engagement with my original question and attempt at solving it gave me enough spark and motivation to continue writing up a full solution.
I've actually almost solved the problem now. Not exactly what you imagined, but fairly close. Maybe someone else can use this as a basis for a better version. Either way, with
{-# LANGUAGE TemplateHaskell, LambdaCase #-}
import Language.Haskell.TH
dump :: ExpQ -> ExpQ
dump tuple =
listE . map dumpExpr . getElems =<< tuple
where
getElems = \case { TupE xs -> xs; _ -> error "not a tuple in splice!" }
dumpExpr exp = [| $(litE (stringL (pprint exp))) ++ " = " ++ show $(return exp)|]
you get the ability to do something like
λ> let x = True
λ> print $(dump [|(not x, x, x == True)|])
["GHC.Classes.not x_1627412787 = False","x_1627412787 = True","x_1627412787 GHC.Classes.== GHC.Types.True = True"]
which is almost what you wanted. As you see, it's a problem that the pprint function includes module prefixes and such, which makes the result... less than ideally readable. I don't yet know of a fix for that, but other than that I think it is fairly usable.
It's a bit syntactically heavy, but that is because it's using the regular [| quote syntax in Haskell. If one wanted to write their own quasiquoter, as you suggest, I'm pretty sure one would also have to re-implement parsing Haskell, which would suck a bit.