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A brief introduction to Haskell

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* Born as an open source vehicle for programming language research
 
* Born as an open source vehicle for programming language research
 
* One of the youngest children of ML and Lisp
 
* One of the youngest children of ML and Lisp
* Particularly useful for manipulating data structures, (i.e. [http://haskell.org/ghc compilers] and [http://pugscode.org interpreters]), and parallel programming
+
* Particularly useful for programs that manipulate data structures (such as [http://haskell.org/ghc compilers] and [http://pugscode.org interpreters]), and for concurrent/parallel programming
   
 
[http://haskell.org/haskellwiki/Old_news#Archives_by_year History]:
 
[http://haskell.org/haskellwiki/Old_news#Archives_by_year History]:
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* [http://haskell.org/hugs Hugs]
 
* [http://haskell.org/hugs Hugs]
 
* [http://haskell.org/haskellwiki/Yhc YHC]
 
* [http://haskell.org/haskellwiki/Yhc YHC]
* [http://repetae.net/computer/jhc/JHC]
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* [http://repetae.net/computer/jhc/JHC JHC]
 
* [http://haskell.org/nhc98 nhc98]
 
* [http://haskell.org/nhc98 nhc98]
 
* [http://www.cs.uu.nl/helium/ Helium]
 
* [http://www.cs.uu.nl/helium/ Helium]
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* Lazy evaluation: results are only computed if they're required (strictness optional)
 
* Lazy evaluation: results are only computed if they're required (strictness optional)
 
* Everything is an expression
 
* Everything is an expression
* Completely higher-order functions: functions can be defined anywhere in the code, passed as arguments, and returned as values.
+
* First-class functions: functions can be defined anywhere, passed as arguments, and returned as values.
 
* Both compiled and interpreted implementations available
 
* Both compiled and interpreted implementations available
 
* Full type inference -- type declarations optional
 
* Full type inference -- type declarations optional
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These are all conceptually more [http://haskell.org/haskellwiki/Research_papers/Type_systems advanced ideas].
 
These are all conceptually more [http://haskell.org/haskellwiki/Research_papers/Type_systems advanced ideas].
   
Compared to similar functional languages, Haskell differs in its support for:
+
Compared to similar functional languages, Haskell differs in that it
  +
has support for:
   
 
* Lazy evaluation
 
* Lazy evaluation
* Pure by default
+
* Pure functions by default
* Monadic effects
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* Monadic side effects
 
* Type classes
 
* Type classes
 
* Syntax based on layout
 
* Syntax based on layout

Revision as of 01:04, 27 October 2006


Inspired by the Introduction to OCaml.

Contents

1 Background

Haskell is:

  • A language developed by the programming languages research community.
  • Is a lazy, purely functional language (that also has imperative features such as side effects and mutable state, along with strict evaluation)
  • Born as an open source vehicle for programming language research
  • One of the youngest children of ML and Lisp
  • Particularly useful for programs that manipulate data structures (such as compilers and interpreters), and for concurrent/parallel programming

History:

  • 1990. Haskell 1.0
  • 1991. Haskell 1.1
  • 1993. Haskell 1.2
  • 1996. Haskell 1.3
  • 1997. Haskell 1.4
  • 1998. Haskell 98
  • 2000-2006. Period of rapid language and community growth
  • ~2007. Haskell Prime

Implementations:

2 Haskell features

Has some novel features relative to Java (and C++).

  • Immutable variables by default (mutable state programmed via monads)
  • Pure by default (side effects are programmed via monads)
  • Lazy evaluation: results are only computed if they're required (strictness optional)
  • Everything is an expression
  • First-class functions: functions can be defined anywhere, passed as arguments, and returned as values.
  • Both compiled and interpreted implementations available
  • Full type inference -- type declarations optional
  • Pattern matching on data structures -- data structures are first class!
  • Parametric polymorphism
  • Bounded parametric polymorphism

These are all conceptually more advanced ideas.

Compared to similar functional languages, Haskell differs in that it has support for:

  • Lazy evaluation
  • Pure functions by default
  • Monadic side effects
  • Type classes
  • Syntax based on layout

The GHC Haskell compiler, in particular, provides some interesting extensions:

  • Generalised algebraic data types
  • Impredicative types system
  • Software transactional memory
  • Parallel, SMP runtime system

3 The Basics

Read the language definition section of the manual to supplement these notes. For more depth and examples see here.

3.1 Interacting with the language

Interacting with Haskell via the GHCi interpreter:

  • expressions are entered at the prompt
  • newline signals end of input

Here is a GHCi sessoin, starting from a UNIX prompt.

   $ ghci
      ___         ___ _
     / _ \ /\  /\/ __(_)
    / /_\// /_/ / /  | |      GHC Interactive, version 6.4.2, for Haskell 98.
   / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
   \____/\/ /_/\____/|_|      Type :? for help.
   Loading package base-1.0 ... linking ... done.
    Prelude> let x = 3 + 4
  • Here the incredibly simple Haskell program
    let x = 3+4
    is
compiled, loaded, and bound to the variable
x
.
Prelude> :t x
x :: Integer

We can ask the system what type it automaticaly inferred for our

variable.
x :: Integer
means that the variable
x
"has type"
Integer
, the type of unbounded

integer values.

Prelude> x
7

A variable evaluates to its value.

Prelude> x + 4
11
The variable
x
is in scope, so we can reuse it in later

expressions.

Prelude> let x = 4 in x + 3
7
Local variables may be bound using
let
, which declares a

new binding for a variable with local scope.

Alternatively, declarations typed in at the top level are like an open-ended let:

Prelude> let x = 4
Prelude> let y = x + 3
Prelude> x * x
16
 
Prelude> :t x
x :: Integer
Prelude> :t y
y :: Integer
Prelude> :t x * x
x * x :: Integer

Notice that type inference infers the correct type for all the expressions, without us having to ever specify the type explicitly.

4 Basic types

There is a range of basic types, defined in the language Prelude

Int         -- bounded, word-sized integers
Integer     -- unbounded integers
Double      -- floating point values
Char        -- characters
String      -- strings
()          -- the unit type
Bool        -- booleans
[a]         -- lists
(a,b)       -- tuples / product types
Either a b  -- sum types
Maybe a     -- optional values

For example:

7
12312412412412321
3.1415
'x'
"haskell"
()
True, False
[1,2,3,4,5]
('x', 42)
Left True, Right "string"
Nothing, Just True

These types have all the usual operations on them, in the standard libraries.

5 Functions (including Patterns and Higher-Order Functions)

6 Declaring our own types

7 Imperative features: monadic IO, references, mutable arrays, exceptions

8 Type classes