CODEWITHSUNDEEP
haskellrecursiontypestuplestype-mismatch
Raphael J.F. Berger
Raphael J.F. Berger

Reputation: 167

Recursion in conditional definition in Haskell gives error ( Occurs check: cannot construct the infinite type:)

I want to define a function conditional and using recursion in Haskell (ghci 7.x) like:

partitions n k m = gfunct n k m
       where
       gfunct n k m
         | n == 0 && k == 0 = 1
         | n < m  || k == 0 = 0
         | otherwise        = gfunct (n-m,k-1,m+1) + gfunct (n,k,m+1)

I get an error:

fib.hs:20:35:
Occurs check: cannot construct the infinite type:
  a1 ~ t6 -> (t5, t6, t7) -> a1
Relevant bindings include
  m :: (t5, t6, t7) (bound at fib.hs:17:23)
  k :: t6 (bound at fib.hs:17:21)
  n :: (t5, t6, t7) (bound at fib.hs:17:19)
  gfunct :: (t5, t6, t7) -> t6 -> (t5, t6, t7) -> a1
    (bound at fib.hs:17:12)
In the first argument of ‘(+)’, namely
  ‘gfunct (n - m, k - 1, m + 1)’
In the expression:
  gfunct (n - m, k - 1, m + 1) + gfunct (n, k, m + 1)

What am I doing wrong?

Upvotes: 1

Views: 112

Answers (1)

Will Ness
Will Ness

Reputation: 71119

A function 

g a b c = 0

will return 0 if supplied by three values, a, b and c; but a function

g (a,b,c) = 0

will return 0 if supplied by one value (a,b,c). Which is a triple of values, a, b and c.

Upvotes: 3

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