[Haskell-beginners] Data.Map, types and debug
legajid
studying IO with files, i got this tutorial program.
By the way, i discover the Data.Map module.
Several questions then come to my mind :
q1: why, in mapM_, make (sort (keys grades)) ? keys is already sorted, isn't it?
q2: why, in mapM_, have grades twice (in draw then keys).
I wonder if writing only draw grades, one could then extract the s and g parts in the draw function (via keys and elems).
From this, i have problems with the type of grades; which is it? The insert function, using insertWith should give
Map k a, but foldr seems to change this (couldn't match expected type [Map k a] against inferred type Map [String] a1
on the mapM_ line .
q3: To solve the types problem, I tried to debug : i can get the types for s, marks and avg but for g, it says not in scope.
How can i get this information ?
q4: I also tried to type the parameter in draw (draw (x::Map k a) = ...) but i got an error :
Illegal signature in pattern
use -XScopedTypeVariables to permit it
Since this parameter is visibly not set by default in ghci, is it a good idea to set it ?
Thanks and happy new year.
Didier
import Data.Char <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Char>
import Data.Maybe <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Maybe>
import Data.List
import Data.Map hiding (map <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map>)
import Text.Printf
main = do
src <- readFile <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:readFile> "grades"
let pairs = map <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map> (split.words <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:words>) (lines <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:lines> src)
let grades = foldr <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:foldr> insert empty pairs
mapM_ <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:mapM_> (draw grades) (sort (keys grades))
where
insert (s, g) = insertWith (++ <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.>) s [g]
split [name,mark] = (name, read <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:read> mark)
draw g s = printf "%s\t%s\tAverage: %f\n" s (show <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:show> marks) avg
where
marks = findWithDefault (error <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:error> "No such student") s g
avg = sum <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:sum> marks / fromIntegral <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:fromIntegral> (length <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:length> marks) :: Double <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Double>
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Daniel Fischer
> Hi,
> studying IO with files, i got this tutorial program.
> By the way, i discover the Data.Map module.
> Several questions then come to my mind :
> q1: why, in mapM_, make (sort (keys grades)) ? keys is already sorted,
> isn't it?
It is. That is also specified in the documentation, so it's superfluous.
It may be a leftover from earlier versions using a data structure which doesn't guarantee
to return the keys in ascending order.
> q2: why, in mapM_, have grades twice (in draw then keys).
draw takes two arguments, a Map in which to look up the marks and a key to look up, the
name of the student. The list of keys over which we want to mapM_ is the list of keys in
the Map, (keys grades). The function we want to mapM_ is (look up marks of student in the
Map grades and then output marks and average), that is (draw grades).
> I wonder if writing only draw grades, one could then extract the s and g
> parts in the draw function (via keys and elems). From this, i have problems
> with the type of grades; which is it? The insert function, using insertWith
> should give Map k a, but foldr seems to change this (couldn't match
> expected type [Map k a] against inferred type Map [String] a1 on the mapM_
> line .
? I don't understand what you tried to do there.
You can write a function draw1 which takes only a Map as argument so that
draw1 grades
is the same as
mapM_ (draw grades) (keys grades)
> q3: To solve the types problem, I tried to debug : i can get the types for
> s, marks and avg but for g, it says not in scope. How can i get this
> information ?
g is a parameter of draw, so there is no entity g defined outside the definition of draw.
To find the type of g, ask ghci:
*Grades> :t draw
draw
:: (PrintfArg k, PrintfType t, Ord k) => Map k [Double] -> k -> t
, since g is the first argument of draw, its type is the type to the left of the first
(top level) '->', namely Map k [Double]
(I have here ignored the type class constraints on k).
> q4: I also tried to type the parameter in draw (draw (x::Map k a) = ...)
> but i got an error : Illegal signature in pattern
> use -XScopedTypeVariables to permit it
> Since this parameter is visibly not set by default in ghci, is it a good
> idea to set it ?
If you need it. Here, you can solve the problem with a type signature on draw, e.g.
draw :: Map String [Double] -> String -> IO ()
>
> Thanks and happy new year.
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