Re: [Haskell-fr] Fractional etc...

6 Nov 2007


      --- Dupont Corentin  wrote:
...
There must be a way to divide a real by an integer??
My integer is eventually a "number of elements" as
in the exemple of Dan.
As you can see below it's impossible to divide a Float
by an Integer directly. Haskell is "strongly-typed" .
But there exist a conversion function:

fromInt :: Int->Float

Prelude> (10.2::Float)/(2::Integer)
ERROR - Type error in application
*** Expression     : 10.2 / 2
*** Term           : 10.2
*** Type           : Float
*** Does not match : Integer

Prelude> (10.2::Float)/(2::Int)
ERROR - Type error in application
*** Expression     : 10.2 / 2
*** Term           : 10.2
*** Type           : Float
*** Does not match : Int

Solution:

Prelude> (10.2::Float)/fromInt(2::Int)
5.1

References:
Simon Thompson, Craft of Functional Programming,
second edition, page 44 - basic types and definitions.
...
How would you compute a mean then?
On 10/29/07, Dan Popa  wrote:
...
Why don't you try :
...
map (/5.0) [0.0 .. 10.0]
let m = 5.0
map (/m) [0.0 .. 10.0]
I have a similar problem trying a divison by a
...
of a list. Finaly I have to build a version of
length
called len:
len [] = 0.0
len (h:t)  = 1.0 + len t
Dan
--- Dupont Corentin 
wrote:
...
Salut,
J'ai vraiment du mal à mettre au point un petit
programme...
Il me sort souvent des problèmes du style :
No instance for (Fractional Integer)
     arising from use of `/' at
<interactive>:1:4-7
  Possible fix: add an instance declaration for
(Fractional Integer)
Par exemple sous ghci si je fait:
map (/5) [0..10]
tout se passe bien.
Mais si je me dit "tiens je voudrais bien
...
...
le 5":
let m = 5
map (/m) [0..10]
il me sort:
    No instance for (Fractional Integer)
      arising from use of `/' at
<interactive>:1:4-7
    Possible fix: add an instance declaration
for
(Fractional Integer)
    In the first argument of `map', namely `(/
m)'
    In the expression: map ((/ m)) ([0 .. 10])
    In the definition of `it': it = map ((/ m))
([0
.. 10])
Pourtant les 2 codes me sembles assez
équivalents!!!
Je peux utiliser des "fromInteger", ce qui
résout
temporairement le problème.
Je me retrouve par la suite avec des (de
mémoire)
Infered type : Int
Expected type : Integer
Ce qui me laisse dans le flou...
a+
Corentin
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