On Thu, Jan 27, 2011 at 14:55, Renzo Orsini wrote:

In studying Haskell, I produced the following output from GHC:

xxx-3:~ xxx$ GHCi GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> let f 7 = "ok" Prelude> let f x = "no" Prelude> f 3 "no" Prelude> f 7 "no"

I suppose it is correct. However, for someone who is interested in the language, it seems very counterintuitive... Somebody would be so kind to explain to a neophyte this "feature" of the language?

I suppose it comes down to pattern matching being on form, not on value. /M -- Magnus Therning                      OpenPGP: 0xAB4DFBA4 email: magnus@therning.org   jabber: magnus@therning.org twitter: magthe               http://therning.org/magnus

On Thursday 27 January 2011 15:55:58, Renzo Orsini wrote:

In studying Haskell, I produced the following output from GHC:

xxx-3:~ xxx$ GHCi GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> let f 7 = "ok" Prelude> let f x = "no" Prelude> f 3 "no" Prelude> f 7 "no"

I suppose it is correct. However, for someone who is interested in the language, it seems very counterintuitive... Somebody would be so kind to explain to a neophyte this "feature" of the language?

It's not a feature of the language, it's a feature of the interpreter. In ghci's own words (evoked by the -Wall flag): Prelude> let f 7 = "ok" <interactive>:1:5: Warning: Pattern match(es) are non-exhaustive In an equation for `f': Patterns not matched: #x with #x `notElem` [7#] Prelude> let f x = "no" <interactive>:1:5: Warning: This binding for `f' shadows the existing binding bound at <interactive>:1:5 <interactive>:1:7: Warning: Defined but not used: `x' You have defined two independent functions, the second overwriting the first binding. To get what you wanted, you have to put the branches on the same line, separated by a semicolon: Prelude> let f 7 = "ok"; f _x = "no" (0.02 secs, 3348380 bytes) Prelude> f 3 "no" (0.01 secs, 1962728 bytes) Prelude> f 7 "ok" (I prefixed the x with an underscore in the second equation to preven an "unused variable" warning, could also have used the wildcard _).

Thank you very much.

Renzo

Cheers, Daniel

You have defined 2 functions that are called f and only the last definition is used (f x = "no"). The definition you want should be written let f 7 = "ok"; f x = "no" so the function is defined once. The problem comes from using the interpreter and not from haskell. On Thu, 2011-01-27 at 15:55 +0100, Renzo Orsini wrote:

In studying Haskell, I produced the following output from GHC:

xxx-3:~ xxx$ GHCi GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> let f 7 = "ok" Prelude> let f x = "no" Prelude> f 3 "no" Prelude> f 7 "no"

I suppose it is correct. However, for someone who is interested in the language, it seems very counterintuitive... Somebody would be so kind to explain to a neophyte this "feature" of the language?

Thank you very much.

Renzo _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 1/27/11 09:55 , Renzo Orsini wrote:

Prelude> let f 7 = "ok" Prelude> let f x = "no"

Doing it in separate let bindings doesn't combine; instead, the second one hides the first. let {f 7 = "ok"; f x = "no"} is more likely to do what you expect. - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk1BpscACgkQIn7hlCsL25Xp2ACfYqe/fkuNY9YSnXM3ptFFGPvD 0k0An0hLeo4w7cv2ALc9MQJRhZIEOtyJ =pYh3 -----END PGP SIGNATURE-----