[Haskell-beginners] beginner's type error

Ivan Moore ivan.r.moore at gmail.com
Thu Mar 26 18:01:07 EDT 2009

Hi all,

consider this very small function:

thing n = n + round(sqrt n)

It loads into ghci with no warnings. When I try to run "thing 10" I get:

*Main> :load c:\temp\statictype.hs
[1 of 1] Compiling Main             ( C:\temp\statictype.hs, interpreted )
Ok, modules loaded: Main.
*Main> thing 10

    Ambiguous type variable `t' in the constraints:
      `Integral t' arising from a use of `thing' at <interactive>:1:0-7
      `RealFrac t' arising from a use of `thing' at <interactive>:1:0-7
      `Floating t' arising from a use of `thing' at <interactive>:1:0-7
    Probable fix: add a type signature that fixes these type variable(s)

I have tried to add various type signatures (without really knowing
what I'm doing!) and haven't been able to get it to work.

I am confused about a few things related to this:
(a) what type signature fixes it and why it needs any help - it looks
like the sort of thing that type inference shouldn't need any help
(b) it looks like a runtime type error and I thought you didn't get
runtime type errors in Haskell
(c) if I substitute 10 for n and do "10 + round(sqrt 10)" I get the
expected answer 13

any help most welcome.



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