The difference between: *Main> return 1 :: Maybe Integer And *Main> return Nothing :: Maybe String Is that 1 has type Integer, but Nothing has type Maybe a. If you want to wrap Nothing into another Maybe value, you should specify the type for Nothing like this: *Main> return Nothing...

haskell,ghc,real-world-haskell

This should work: ghc -o simple Main.hs SimpleJSON.hs Or even something like this should work: ghc -c SimpleJSON.hs ghc -c Main.hs ghc -o simple Main.o SimpleJSON.o Or as @chi points out, you can use this: ghc --make SimpleJSON.hs Main.hs ...

haskell,real-world-haskell,ppm

Netpbm contains tools to convert popular formats to pbm. pngtopnm < file.png | ppmtopgm | pgmtopbm > file.ppm pngtopnm may convert directly to pgm or ppm it depends on the the colour-space of the input file. you may need pnmtoplainpnm on the end of the pipeline if your program is...

regex,haskell,haskell-platform,real-world-haskell

According to the Haskell Wiki (https://wiki.haskell.org/Regular_expressions#regex-base): Executive summary: If you want a bug-free and/or portable POSIX extended regular expression library to use from Haskell, then regex-posix will not help you. You should use the regex-tdfa package instead. Try using the tdfa regex package from http://hackage.haskell.org/package/regex-tdfa. If you are not familiar...

haskell,recursion,real-world-haskell

So I think what you're saying is that you know how to recursively append to a list by doing something like foobar :: [x] -> [y] foobar ( []) = [] foobar (x:xs) = {- ...stuff... -} : foobar xs but you have no idea how to do that with...

xs' would be bound to the string "ello". xs would be bound to the string "hello". The @ pattern lets you give a name to a variable while also matching its structure and possibly giving name to the components....

haskell,monads,real-world-haskell

Your code is very similar to the Writer [a] monad, and I don't think there is a simpler way of fixing it than closing the gap in all but naming. Specifically: Since a monad must allow an arbitrary result type, the type of the effect part cannot depend on the...

You must carry generator into pure process (chaining new random generator state) randomR_nTimes_rec :: (RandomGen g, Random a) => Int -> (a, a) -> g -> ([a], g) randomR_nTimes_rec 0 _ g = ([], g) randomR_nTimes_rec n i g = (x:xs, g'') where ( x, g' ) = randomR i...

In Haskell source files, top-level definitions like f shouldn't be introduced with a let - just write f [] = [] f ('\n':',':a) = f ('\n' : a) f (a:b) = a : f b Also you need make sure that the left-hand side of each clause in a definition...

There is an interaction going on between two instances of readsPrec: the readsPrec you defined for instance Read Color the readsPrec defined by the Prelude for instance Read [a] Let's call the readsPrec in #2 readsPrecList. When evaluating read "[Red]" :: [Color] what first happens is the readsPrecList is called....

The relevant definitions are available in the Report, and look like this: read :: (Read a) => String -> a read s = case [x | (x,t) <- reads s, ("","") <- lex t] of [x] -> x [] -> error "Prelude.read: no parse" _ -> error "Prelude.read: ambiguous parse"...

What @simonzack is alluding to is that you should write listlen and sumx as folds. Here is listlen written as a fold: listlen :: [a] -> Int listlen xs = go 0 xs -- 0 = initial value of accumulator where go s [] = s -- return accumulator go...

haskell,concurrency,parallel-processing,mapreduce,real-world-haskell

In this line: result = mapReduce Control.Parallel.Strategies.parMap stringToWordCountMap Control.Parallel.Strategies.parList reduceWordCountMaps fileInLines for a Strategy you should provide a value like rpar or rseq. See this documentation for other defined strategies. So the above line should look like: result = mapReduce rpar stringToWordCountMap rpar reduceWordCountMaps fileInLines I also got some indentation...

The recommended time functions have changed. Data.Time etc. from the time package is a complete and more logical rewrite of Haskell's time functions. To quote the top of http://hackage.haskell.org/package/old-time-1.1.0.2/docs/System-Time.html: The standard time library from Haskell 98. This library is deprecated, please look at Data.Time in the time package instead. ...