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In Haskell, indentation can be used in several syntactic constructs, including do, let, case, class, and instance. The use of indentation to indicate program structure originates in Peter J. Landin 's ISWIM language, where it was called the off-side rule .
If the second sub-expression can be a further simple conditional expression, we can give more alternatives to try before the last fall-through: (x>0) -> 1/x; (x<0) -> -1/x; 0 In 1966 ISWIM had a form of conditional expression without an obligatory fall-through case, thus separating guard from the concept of choosing either-or.
1 2 3 Case-expressions in Haskell and match-expressions in F# and Haskell allow both switch-case and pattern matching usage. ^ In a Ruby case construct, regular expression matching is among the conditional flow-control alternatives available.
Haskell features lazy evaluation, lambda expressions, pattern matching, list comprehension, type classes and type polymorphism. It is a purely functional programming language, which means that functions generally have no side effects .
Hugs deviates from the Haskell 98 specification [2] in several minor ways. [3] For example, Hugs does not support mutually recursive modules. A list of differences exists. [4] The Hugs prompt is a Haskell read–eval–print loop (REPL). It accepts expressions for evaluation, but not module, type, or function definitions.
Generalised Algebraic Datatype Page on the Haskell wiki; Generalised Algebraic Data Types in the GHC Users' Guide; Generalized Algebraic Data Types and Object-Oriented Programming; GADTs – Haskell Prime – Trac; Papers about type inference for GADTs, bibliography by Simon Peyton Jones; Type inference with constraints, bibliography by Simon ...
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In monadic Haskell, return is the standard name for unit, plus lambda expressions must be handled explicitly, but even with these technicalities, the Maybe monad makes for a cleaner definition: add mx my = mx >>= ( \ x -> my >>= ( \ y -> return ( x + y )))