[Haskell-cafe] Question about data

[Paul Reiners]

I've created a simple type declaration:

data MathExpression = Float
| Add MathExpression MathExpression
| Subtract MathExpression MathExpression
| Multiply MathExpression MathExpression
| Divide MathExpression MathExpression
  deriving (Show)

Now how do I create an instance of MathExpression which is just a Float?

This doesn't work:

*Main let pi = 3.14 :: MathExpression

interactive:1:10:
No instance for (Fractional MathExpression)
  arising from the literal `3.14'
Possible fix:
  add an instance declaration for (Fractional MathExpression)
In the expression: 3.14 :: MathExpression
In an equation for `pi': pi = 3.14 :: MathExpression
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[Haskell-cafe] Couldn't match expected type `Integer' against inferred type `Int'

[Paul Reiners]

Why am I getting this error:

 Couldn't match expected type `Integer' against inferred type `Int'
 In the expression: foldl step 0 xs
 In the definition of `asInt_foldAux':
 asInt_foldAux xs
 = foldl step 0 xs
 where
 step acc '.' = error cannot handle decimal 
 numbers
 step acc x = acc * 10 + digitToInt x

for this code?

 import Data.Char (digitToInt)
 
 asInt_fold :: String - Integer
 asInt_fold ('-':xs) = -(asInt_foldAux xs)
 asInt_fold xs   = asInt_foldAux xs
 
 asInt_foldAux :: String - Integer
 asInt_foldAux xs = foldl step 0 xs
where step acc '.' = error cannot handle decimal numbers
  step acc x = acc * 10 + digitToInt x

Note that I'm using Int, rather than Integer, to avoid silent overflow errors.

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Re: [Haskell-cafe] Couldn't match expected type `Integer' against inferred type `Int'

[Paul Reiners]

On Aug 8, 2011, at 7:59 PM, Ivan Lazar Miljenovic wrote:

 On 9 August 2011 10:49, Paul Reiners paul.rein...@gmail.com wrote:
 Why am I getting this error:
 
 Couldn't match expected type `Integer' against inferred type `Int'
 In the expression: foldl step 0 xs
 In the definition of `asInt_foldAux':
 asInt_foldAux xs
 = foldl step 0 xs
 where
 step acc '.' = error cannot handle decimal
 numbers
 step acc x = acc * 10 + digitToInt x
 
 for this code?
 
 import Data.Char (digitToInt)
 asInt_fold :: String - Integer
 asInt_fold ('-':xs) = -(asInt_foldAux xs)
 asInt_fold xs   = asInt_foldAux xs
 asInt_foldAux :: String - Integer
 asInt_foldAux xs = foldl step 0 xs
where step acc '.' = error cannot handle decimal numbers
  step acc x = acc * 10 + digitToInt x
 
 digitToInt returns an Int; as such the result of asInt_foldAux is an
 Int, but you've specified in its type signature that it should be
 returning an Integer.

So is there something like digitToInteger?  Or can I somehow cast the Int to an 
Integer?

 
 Note that I'm using Int, rather than Integer, to avoid silent overflow
 errors.
 
 Should that be the other way round?

Yes, it should be the other way round.


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[Haskell-cafe] Binding of an integer variable

[Paul Reiners]

I have a question about the following GHCi interaction:

Prelude let x = 23
Prelude :show bindings
x :: Integer = _

What is the meaning of the underscore in the third line?  Why doesn't it say
this, instead?

x :: Integer = 23
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[Haskell-cafe] Input redirection in WinGHCi

[Paul Reiners]

I know I can do the following from a command prompt:

$ runghc WC  quux.txt

How do I do this in WinGHCi? I know I have to first load the file like this:

Prelude :load WC

But then what? This doesn't work:

*Main WC  quux.txt

interactive:1:1: Not in scope: data constructor `WC'

interactive:1:6: Not in scope: `quux'

interactive:1:11: Not in scope: `txt'
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