I try the follow line in hugs (mode 98 enable)
Prelude> :t (let f = \x -> x == x in ( f 'a', f 123 ) )
ERROR: Illegal Haskell 98 class constraint in inferred type
*** Expression : let {...} in (f 'a',f 123)
*** Type : Num Char => (Bool,Bool)
Prelude>
This happend because Num Char can never be satisfied with a
predicate evidence. However in hugs without 98 mode the result
of the previos line is :
Prelude> :t ( let f = \x -> x == x in (f 'a' , f 123 ) )
let {...} in (f 'a',f 123) :: Num Char => (Bool,Bool)
This (maybe) indicates how hugs make evidence elimination in
runtime while Haskell 98 donīt do that. I donīt have a clear
idea about this, please someone can help me?. Thanks.
Alejandro Russo
Teacher in Computer Science
Rosario National University
Argentina
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