Question

Trying to use hmatrix, to create a zero marix.
For some reason, when I try this on command line, it works:

buildMatrix 2 3 (\(r,c) -> fromIntegral 0)

However, when I try to do the same thing in my code:

type Dim = (Int, Int)

buildFull :: Matrix Double -> Vector Int -> Vector Int -> Dim -> Int
buildFull matrix basic nonbasic (m, n) = do
    -- Build mxn matrix of zeroes
    let f = buildMatrix m n (\(r,c) -> fromIntegral 0)
    m

it fails:

Pivot.hs:23:17:
    Ambiguous type variable `a0' in the constraints:
      (Element a0) arising from a use of `buildMatrix'
                   at Pivot.hs:23:17-27
      (Num a0) arising from a use of `fromIntegral' at Pivot.hs:23:44-55
    Probable fix: add a type signature that fixes these type variable(s)
    In the expression: buildMatrix m n (\ (r, c) -> fromIntegral 0)
    In an equation for `f':
        f = buildMatrix m n (\ (r, c) -> fromIntegral 0)
    In the expression:
      do { let f = buildMatrix m n (\ (r, c) -> ...);
           m }
Failed, modules loaded: none.

Answer 1

type Dim = (Int, Int)

buildFull :: Matrix Double -> Vector Int -> Vector Int -> Dim -> Int
buildFull matrix basic nonbasic (m, n) = do
    -- Build mxn matrix of zeroes
    let f = buildMatrix m n (\(r,c) -> fromIntegral 0)
    m

First, to use do-notation, you need a monadic return type, so that won’t compile even after fixing the ambiguous element type (as I was reminded by @Carl, it would be okay here while there’s only a single expression so that no (>>=) or (>>) is needed).

Concerning the element type, in the let-binding, there is no way to find out which type to use, whether fromIntegral should return Double, Integer or whatever. Often the type to be used can be inferred from context, by the expressions it is used in. Here, f is nowhere used, so there’s no context. Hence in this situation, you have to specify the type by a signature, that can be

let f :: Matrix Double
    f = buildMatrix m n (const 0)

or

let f = buildMatrix m n (\_ -> (0 :: Double))

if you want the element type o be Double.