I cannot figure out how to make the concise if-then-else notation work, mentioned at [ http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse ]. This works,
import System.Environment
main = do
args <- getArgs
if (args !! 0) == "hello"
then
print "hello"
else
print "goodbye"
but this does not, and inserting said semicolons (see link) just result in parse errors for me.
import System.Environment
main = do
args <- getArgs
if (args !! 0) == "hello" then
print "hello"
else
print "goodbye"
The link you provided describes a proposal, which sounds like it is not part of the Haskell standard (although the link mentions that it's implemented in jhc, GHC and Hugs). It's possible that the version of the Haskell compiler you're using, or the set of flags you're using, does not allow for the optional-semicolon behavior described in the link.
Try this:
import System.Environment
main = do
args <- getArgs
if (args !! 0) == "hello" then
print "hello"
else
print "goodbye"
In Haskell 98 “if … then … else …” is a single expression. If it’s split to multiple lines, the ones following the first one must be indented further.
Just like the following is wrong…
do
1 +
2
…and the following works…
do
1 +
2
…the following is also wrong…
do
if True then 1
else 2
…and the following works.
do
if True then 1
else 2
As the other comments already mention, Haskell 2010 allows the “then” and “else” parts on the same level of indentation as the “if” part.
Haskell syntax and language are extended though {-# LANGUAGE ... #-} pragmas at the start of the source files. The DoAndIfThenElse extension is recognized since it is one of those listed in the Cabal documentation. Current GHC enables this by default.
I usually indent the else one space more than the if. Unless then whole if fits nicely on a single line.
Related
I'm a newbie in Haskell and I'm lost. I was trying to parse a math expression, but really don't know how Haskell programming works well. So what I'm trying to write is a program to resolve a simple math expression. I'm looking for ideas on how I could resolve by giving arguments.
The command line could look like : ./math "3 + 2" or ./math "5 * 8"
My code looks like this:
import System.Environment (getArgs)
import Text.Printf
main :: IO ()
main = do
args <- getArgs
printf "%.2f" args[1] + args[2]
Haskell has no array[index] syntax. It does have list!!index syntax (which isn't really special syntax at all, !! is just an infix-function defined in the prelude). Note that Haskell indices are 0-based and unlike in Bash, the zeroth argument is not the command name itself, so you probably want indices 0 and 1.
Also, in Haskell function application binds more tightly than any operators. So, if you were to write
printf "%.2f" args!!0 + args!!1
it would parse as ((printf "%.2f" args)!!0) + (args!!1), which is obviously not right. You need to make explicit what precedence you want:
printf "%.2f" (args!!0 + args!!1)
or as we like to do it, with $ instead of parens:
printf "%.2f" $ args!!0 + args!!1
That's still not right, because the arguments come in as strings, but the addition should be performed on numbers. For this, you need to read the numbers; I'd suggest you do that separately:
import Text.Read (readMaybe)
main = do
args <- getArgs
let a, b :: Double
Just a = readMaybe $ args!!0
Just b = readMaybe $ args!!1
printf "%.2f" $ a + b
$ runhaskell Argsmath.hs 3 2
5.00
Of course this will not allow you to do stuff like ./math "5 * 8" because you have no means of parsing the *. For that, something read-based would be awkward; I suggest you check out parser combinator libraries, there are plenty of tutorials around; this one seems to be nice and simple.
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.
I'm having problems with an exercise, and can not understand the error.
It should be a simple exercise with args:
import System.IO
import System.Environment
main= do
args < - getArgs
nomeficheiro <- return( args !! 0)
putStrnLn ( "Name is" ++ nomeficheiro)
Then i should run it, with : $ ./comando James
The error:
<interactive>:51:1:
parse error on input ‘$’
Perhaps you intended to use TemplateHaskell
I've read other doubts about args at this fórum and I didn't find any answer that could help me
$ ./comando James isn't meant to be run on GHCi. Instead, $ at the start of the line indicates that the following line should be run in your bash/cmd/shell, not in GHCi:
# in your favourite shell, in the correct directory
./comando James
If you want to run main with arguments within GHCi, you can use :main args:
ghci> :main James
Further remarks
Your current code isn't indented correctly, so make sure that you fix this too. Also, you can use let nomeficheiro = head args instead of … <- return …. Keep in mind that this could lead to problems if one doesn't supply any argument to your program, since head [] calls error.
I'm trying to write a function in Haskell which checks for some things and then recurses based on some minimal user input. In order to do that I think I have to use do blocks.
cip :: [Argument] -> [Argument] -> Bool -> Bool -> IO()
cip (a:args) pargs burden gameover = do
let nasko = a:pargs
putStrLn (getPremise a)
let newgraph = Carneades.mkArgGraph nasko
let newcaes = (CAES (newgraph,audience2,assStandarts))
let answer = (acceptable (mkProp (getPremise a)) newcaes )
print answer
if(answer==True)
then (cip args nasko burden gameover)
else do
print "One of the arguments is not proved. Here are the premises that need proving"
print (propsForFixing newcaes a)
print "Let's see what you have for the first Propositon"
--add an if to check if no applicable arguments.
print (argumentScanHelp (head (propsForFixing newcaes a)) args)
print "\n Would you like me to apply the firt one? Y/N"
choice <- getLine
if(choice=="Y") then do print "applying the argument"
let applicabee = head (argumentScanHelp (head (propsForFixing newcaes a)) args)
print "Argument targeted"
let newargs = delete applicabee args
let newpargs = applicabee:nasko
print "Argument applied sucsessfuly. Recusing again"
(cip newargs newpargs burden gameover)
return()
It hurts my eyes just by looking at it, but that's do blocks for you.
Everything up to the third do block is okay. But then on this line:
if(choice=="Y") then do print "applying the argument"
let applicabee = head (argumentScanHelp (head (propsForFixing newcaes a)) args)
The complier starts crying:
Main.hs:209:73: parse error on input `let'
Tried a all kinds of different indentations but I can't seem to get it to work.
I don't want to use separate functions, because that means I'll have to pass a lot of arguments around constantly.
Can anyone help me get it right? Also an explanation of what exactly the specifications for nested do block are would be greatly appreciated.
The cause of the error I believe is the misuse of the if expression. You use it as if it were an if statement that exists in most imperative languages. Simply put there must always be an else.
However, in do blocks it makes sense to "not have an else", something like an if statement without an else. Luckily the Control.Monad module will provide you with a function for exactly that:
import Control.Monad (when)
(...)
when (choice=="Y") $ do print "applying the argument"
let applicabee = ...
You seem to already use nested do blocks in the correct way which is good, which basically is that you must indent properly.
PS. Also make sure your last return () is indented like the rest of your code! DS.
Haskell Stack Overflow layout preprocessor
module StackOverflow where -- yes, the source of this post compiles as is
Skip down to What to do to get it working if you want to play with this first (1/2 way down).
Skip down to What I would like if I witter on a bit and you just want to find out what help I'm seeking.
TLDR Question summary:
Can I get ghci to add filename completion to the :so command I defined in my ghci.conf?
Could I somehow define a ghci command that returns code for compilation instead of returning a ghci command, or
does ghci instead have a better way for me to plug in Haskell code as a
file-extension-specific pre-processor, so :l would work for .hs and .lhs files as usual, but use my handwritten preprocessor for .so files?
Background:
Haskell supports literate programming in .lhs source files, two ways:
LaTeX style \begin{code} and \end{code}.
Bird tracks: Code starts with > , anything else is a comment.
There must be a blank line between code and comments (to stop trivial accidental misuse of >).
Don't Bird tracks rules sound similar to StackOverflow's code blocks?
References: 1. The .ghci manual
2. GHCi haskellwiki
3. Neil Mitchell blogs about :{ and :} in .ghci
The preprocessor
I like writing SO answers in a text editor, and I like to make a post that consists of code that works,
but end up with comment blocks or >s that I have to edit out before posting, which is less fun.
So, I wrote myself a pre-processor.
If I've pasted some ghci stuff in as a code block, it usually starts with * or :.
If the line is completely blank, I don't want it treated as code, because otherwise
I get accidental code-next-to-comment-line errors because I can't see the 4 spaces I accidentally
left on an otherwise blank line.
If the preceeding line was not code, this line shouldn't be either, so we can cope with StackOverflow's
use of indentation for text layout purposes outside code blocks.
At first we don't know (I don't know) whether this line is code or text:
dunnoNow :: [String] -> [String]
dunnoNow [] = []
dunnoNow (line:lines)
| all (==' ') line = line:dunnoNow lines -- next line could be either
| otherwise = let (first4,therest) = splitAt 4 line in
if first4 /=" " --
|| null therest -- so the next line won't ever crash
|| head therest `elem` "*:" -- special chars that don't start lines of code.
then line:knowNow False lines -- this isn't code, so the next line isn't either
else ('>':line):knowNow True lines -- this is code, add > and the next line has to be too
but if we know, we should keep in the same mode until we hit a blank line:
knowNow :: Bool -> [String] -> [String]
knowNow _ [] = []
knowNow itsCode (line:lines)
| all (==' ') line = line:dunnoNow lines
| otherwise = (if itsCode then '>':line else line):knowNow itsCode lines
Getting ghci to use the preprocessor
Now we can take a module name, preprocess that file, and tell ghci to load it:
loadso :: String -> IO String
loadso fn = fmap (unlines.dunnoNow.lines) (readFile $ fn++".so") -- so2bird each line
>>= writeFile (fn++"_so.lhs") -- write to a new file
>> return (":def! rso (\\_ -> return \":so "++ fn ++"\")\n:load "++fn++"_so.lhs")
I've used silently redefining the :rso command becuase my previous attemts to use
let currentStackOverflowFile = .... or currentStackOverflowFile <- return ...
didn't get me anywhere.
What to do to get it working
Now I need to put it in my ghci.conf file, i.e. in appdata/ghc/ghci.conf
as per the instructions
:{
let dunnoNow [] = []
dunnoNow (line:lines)
| all (==' ') line = line:dunnoNow lines -- next line could be either
| otherwise = let (first4,therest) = splitAt 4 line in
if first4 /=" " --
|| null therest -- so the next line won't ever crash
|| head therest `elem` "*:" -- special chars that don't start lines of code.
then line:knowNow False lines -- this isn't code, so the next line isn't either
else ('>':line):knowNow True lines -- this is code, add > and the next line has to be too
knowNow _ [] = []
knowNow itsCode (line:lines)
| all (==' ') line = line:dunnoNow lines
| otherwise = (if itsCode then '>':line else line):knowNow itsCode lines
loadso fn = fmap (unlines.dunnoNow.lines) (readFile $ fn++".so") -- convert each line
>>= writeFile (fn++"_so.lhs") -- write to a new file
>> return (":def! rso (\\_ -> return \":so "++ fn ++"\")\n:load "++fn++"_so.lhs")
:}
:def so loadso
Usage
Now I can save this entire post in LiterateSo.so and do lovely things in ghci like
*Prelude> :so StackOverflow
[1 of 1] Compiling StackOverflow ( StackOverflow_so.lhs, interpreted )
Ok, modules loaded: StackOverflow.
*StackOverflow> :rso
[1 of 1] Compiling StackOverflow ( StackOverflow_so.lhs, interpreted )
Ok, modules loaded: StackOverflow.
*StackOverflow>
Hooray!
What I would like:
I would prefer to enable ghci to support this more directly. It would be nice to get rid of the intermediate .lhs file.
Also, it seems ghci does filename completion starting at the shortest substring of :load that determines
you're actually doing load, so using :lso instead of :so doesn't fool it.
(I would not like to rewrite my code in C. I also would not like to recompile ghci from source.)
TLDR Question reminder:
Can I get ghci to add filename completion to the :so command I defined in my ghci.conf?
Could I somehow define a ghci command that returns code for compilation instead of returning a ghci command, or
does ghci instead have a better way for me to plug in Haskell code as a
file-extension-specific pre-processor, so :l would work for .hs and .lhs files as usual, but use my handwritten preprocessor for .so files?
I would try to make a standalone preprocessor that runs SO preprocessing code or the standard literary preprocessor, depending on file extension. Then just use :set -pgmL SO-preprocessor in ghci.conf.
For the standard literary preprocessor, run the unlit program, or use Distribution.Simple.PreProcess.Unlit.
This way, :load and filename completion just work normally.
GHCI passes 4 arguments to the preprocessor, in order: -h, the label, the source file name, and the destination file name. The preprocessor should read the source and write to the destination. The label is used to output #line pragmas. You can ignore it if you don't alter the line count of the source (i.e. replace "comment" lines with -- comments or blank lines).