Help ? :)

[Hal Daume III]

Hi,

You're pretty close, actually.  In general, remember the following:

'type' introduces a type synonym, which means that you're just renaming an
existing type, not creating a new one.  this means that you don't need
constructors.

'data' means that you're defining a new type; in order to do this, you
need to specify constructors.

for instance:

> type Position =3D Int

here, we've just given Int a new name, Position.

> data Bool =3D True | False

here we've defined a new type (Bool) with two constructors, True and
False.

or, if the constructors take arguments:

> data PairOfInts =3D PairOfInts Int Int

here, the type is called PairOfInts and the constructor is called
PairOfInts; the constructor takes two ints as arguments.

HTH,

 - Hal

--
 Hal Daume III                                   | hdaume at isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume

On Mon, 3 Nov 2003, Patty Fong wrote:

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> Hello, I'm fairly new to haskell and functional programming and i'm still
> trying to get my head around certain concepts.
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> I'm wondering if anyone can help me convert this abstract syntax into
> Haskel data and type declarations:
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> <prolog> ::=3D (<assertion> ".")*<?xml:namespace prefix =3D o ns =3D
> "urn:schemas-microsoft-com:office:office" />
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> <assertion> :: =3D <structure> | <rule>
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> <rule> ::=3D <structure> ":-" <structure>("," <structure>)*
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> <structure> ::=3D <name> [=93(=93 <term> (=93,=94 <term>)* =93)=94]
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> <term> ::=3D <number> | <variable> | <structure>
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> <variable> ::=3D <name>
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> =20
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> <name> is simply a String and <number> an Int
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> =20
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> This was my attempt feeble attempt but i ran into numerous errors... :
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> =20
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> type Prolog =3D Assertion
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> data Assertion =3D Structure | Rule
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> type Rule =3D Structure Structure (Structure)
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> type Structure =3D Name [ Term (Term)]
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> data Term =3D Number | Variable | Structure
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> type Variable =3D String
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> type Name =3D String
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> type Number =3D Int
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> =20
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> Any help would be appreciated
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> TIA
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> Patrick.
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