Re: [Haskell-cafe] What monad am I in?
Henry Laxen wrote: Have I, like Monsier Jourdain, been running in the IO monad all my life, and didn't even know it? Marc Weber wrote: Sure... But the ghci error message is another one: Try this: :set -XNoMonomorphismRestriction And I highly recommend putting that line in your .ghci file. There is controversy about whether MR is helpful in general. It is on by default, so I just leave it that way, and it seems to be fine. But at the GHCi prompt, MR is definitely a nuisance. Get rid of it there by using :set in your .ghci file. Regards, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] What monad am I in?
Dear Group, When I fire up ghci and define: increment x = return (x+1) I can say: Main increment 1 and ghci dutifully replies 2. Also as expected, the type signature of increment is: (Num a, Monad m) = a - m a However, if I say: Main let a = increment 1 I get: interactive:1:8: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `increment' at interactive:1:8-18 Probable fix: add a type signature that fixes these type variable(s) Have I, like Monsier Jourdain, been running in the IO monad all my life, and didn't even know it? Thanks, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What monad am I in?
On Tue, 2008-09-02 at 20:25 +, Henry Laxen wrote: Dear Group, When I fire up ghci and define: increment x = return (x+1) I can say: Main increment 1 and ghci dutifully replies 2. Also as expected, the type signature of increment is: (Num a, Monad m) = a - m a However, if I say: Main let a = increment 1 I get: interactive:1:8: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `increment' at interactive:1:8-18 Probable fix: add a type signature that fixes these type variable(s) Have I, like Monsier Jourdain, been running in the IO monad all my life, and didn't even know it? Yes. This is a peculiarity of GHCi (and ghc -e) --- IO actions at top-level are executed by default, while non-IO values are simply printed out. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What monad am I in?
Have I, like Monsier Jourdain, been running in the IO monad all my life, and didn't even know it? Sure, just try readFile doesnotexist within ghci :-) That's an IO action. on the other side ghci (3+7) 10 is no IO action. So I think ghci has two default behaviours differing. Either its a monad, than use IO, else evaluate the result. In both cases show it. The ghc manual sould tell you all about this (too lazy to look it up) But the ghci error message is another one: Try this: :set -XNoMonomorphismRestriction let increment x = return (x+1) let a = increment 1 the line let a = requires to find out about the type of m (Maybe any Monad such as Maybe, list etc) without XNoMonomorphismRestriction. With XNoMonomorphismRestriction you can tell ghc that you don't care yet about this and it should try to resolve m later. That's why ghci shows this: ghci :t a a :: (Num t, Monad m) = m t The second way to get rid of the ghci error message is telling ghci which monad you want: increment 1 -- this works because it's run within IO print $ increment 1 -- won't, because ghc does'n know about the m type print $ fromJust $ increment 1 -- works again, because you tell ghc that m is Maybe here print $ (increment 1 :: [Int]) -- works as well, using list monad ... Sincerly Marc Weber ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What monad am I in?
ghci has some crazy defaulting rules for expressions at the top level. In particular, it tries to unify those expressions with a few different types, including IO. On the other hand, the let-expression is typed like regular Haskell and you run into the monomorphism restriction. -- ryan On Tue, Sep 2, 2008 at 1:25 PM, Henry Laxen [EMAIL PROTECTED] wrote: Dear Group, When I fire up ghci and define: increment x = return (x+1) I can say: Main increment 1 and ghci dutifully replies 2. Also as expected, the type signature of increment is: (Num a, Monad m) = a - m a However, if I say: Main let a = increment 1 I get: interactive:1:8: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `increment' at interactive:1:8-18 Probable fix: add a type signature that fixes these type variable(s) Have I, like Monsier Jourdain, been running in the IO monad all my life, and didn't even know it? Thanks, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What monad am I in?
On Tue, Sep 2, 2008 at 5:33 PM, Ryan Ingram [EMAIL PROTECTED] wrote: ghci has some crazy defaulting rules for expressions at the top level. In particular, it tries to unify those expressions with a few different types, including IO. On the other hand, the let-expression is typed like regular Haskell and you run into the monomorphism restriction. Right. Just to make it clear for the original poster, this monomorphism restriction is not about GHCi specifically, just GHC in general. With the -fno-monomorphism-restriction, you will not get this error. -- ryan On Tue, Sep 2, 2008 at 1:25 PM, Henry Laxen [EMAIL PROTECTED] wrote: Dear Group, When I fire up ghci and define: increment x = return (x+1) I can say: Main increment 1 and ghci dutifully replies 2. Also as expected, the type signature of increment is: (Num a, Monad m) = a - m a However, if I say: Main let a = increment 1 I get: interactive:1:8: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `increment' at interactive:1:8-18 Probable fix: add a type signature that fixes these type variable(s) Have I, like Monsier Jourdain, been running in the IO monad all my life, and didn't even know it? Thanks, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
