Question

The below snippets of code are part of an attempt at creating a function generateUpTo which generates a list pAllSorted which depends on nmax and thus rmax.

nmax = rmax `div` 10

pass = rmax `elem` mot
fail = rmax `notElem` mot

generateUpTo rmax = check rmax
where 
         check pass = pAllSorted
         check fail = error "insert multiple of 10!"

However, when attempting to compile, the compiler gives a “Not in scope” error about rmax in (what is here) line 1,3 and 4.

(How) can I leave rmax undefined until using the generateUpTo function?

Answer 1

If you want to use rmax inside nmax, pass, and fail without passing it as an arguement, you’ll need to include it in the where block of generateUpTo. Otherwise, it’s literally, “not in scope”. Example:

generateUpTo rmax = check rmax
    where 
         check pass = pAllSorted
         check fail = error "insert multiple of 10!"
         nmax = rmax `div` 10
         pass = rmax `elem` mot
         fail = rmax `notElem` mot

If you want these functions to be used in multiple places, you could just accect rmax as an arguement:

nmax rmax = rmax `div` 10
pass rmax = rmax `elem` mot
fail rmax = rmax `notElem` mot

Note – it looks like you also have some problems with your definition of check… the pass and fail value there are just arguements of check, and not the functions you’ve defined above.

Update

to use nmax (the outside-the-where-block scope version), you’ll need to pass the value of rmax to it. Like so:

nmax rmax  -- function application in Haskell is accomplished with a space,
           -- not parens, as in some other languages.

Note, however, the name rmax in the definition of nmax is no longer significant. These functions are all exactly the same:

nmax rmax = rmax `div` 10
nmax a = a `div` 10
nmax x = x `div` 10

Likewise, you don’t need to call it with a value named rmax.

nmax rmax
nmax 10    -- this is the same, assuming rmax is 10
nmax foo   -- this is the same, assuming foo has your 'rmax' value.