Re: [Haskell-cafe] Prolog-style list syntax?

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Ok so I implemented both ideas:

>     elem2 _ []       = False
>    elem2 x [y,: ys] = x == y || elem2 x ys

>    let [x,y,z,w,: ws] = "hello there" in ws -- yields "o there"

In fact it was easier to implement both ideas than make a special case of
`(:)`. So these work

>    let [x,:+ xs] = [1,2,3,:+ Nily] in xs

> let [x,`ConsSet` xs] = [1,2,3,`ConsSet` NilSet] in xs

with decls

>    infixr 5 :+
>    data Listy a = Nily | a :+ (Listy a)  deriving (Eq, Show, Read)

>     infixr 5 `ConsSet`
>    data Set a = NilSet | ConsSet a (Set a)  deriving (Eq, Show, Read)


On Wed, 30 Jun 2021 at 19:00, Anthony Clayden <[email protected]>
wrote:

> Ok thank you for the feedback, I get the message not to re-purpose
> currently valid syntax.
>
>
> >    [x, y ,: ys ]           -- ? not currently valid
>
> We could make list syntax work harder
>
> >    [x, y ,:+ ys ]
>
> Means desugar the commas by applying constructor `:+` instead of `:`. That
> could be in general any constructor starting `:`.
>
> Or indeed could be a pattern synonym starting `:`, which is a 'smart
> constructor' to build the list maintaining some invariant.
>
>
>

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<div dir=3D"ltr"><div>Ok so I implemented both ideas:</div><div><br></div><=
div>&gt;=C2=A0 =C2=A0=C2=A0<span style=3D"font-family:Arial,Helvetica,sans-=
serif">=C2=A0elem2 _ [] =C2=A0 =C2=A0 =C2=A0 =3D False</span></div>&gt;=C2=
=A0 =C2=A0 elem2 x [y,: ys] =3D x =3D=3D y || elem2 x ys<div><font face=3D"=
Arial, Helvetica, sans-serif"><br></font></div><div><font face=3D"Arial, He=
lvetica, sans-serif">&gt;=C2=A0 =C2=A0 let [x,y,z,w,: ws] =3D &quot;hello t=
here&quot; in ws -- yields &quot;o there&quot;</font></div><div><font face=
=3D"Arial, Helvetica, sans-serif"><br></font></div><div><font face=3D"Arial=
, Helvetica, sans-serif">In fact it was easier to implement both ideas than=
 make a special case of `(:)`. So these work</font></div><div><font face=3D=
"Arial, Helvetica, sans-serif"><br></font></div><div><font face=3D"Arial, H=
elvetica, sans-serif">&gt;=C2=A0 =C2=A0=C2=A0</font>let [x,:+ xs] =3D [1,2,=
3,:+ Nily] in xs</div><div><br></div><div>&gt; let [x,`ConsSet` xs] =3D [1,=
2,3,`ConsSet` NilSet] in xs<br><br>with decls</div><div><br></div><div>&gt;=
<span style=3D"font-family:Arial,Helvetica,sans-serif">=C2=A0 =C2=A0 infixr=
 5 :+</span></div>&gt;=C2=A0 =C2=A0 data Listy a =3D Nily | a :+ (Listy a) =
=C2=A0deriving (Eq, Show, Read)<br><div><br></div><div>&gt;=C2=A0=C2=A0 =C2=
=A0 infixr 5 `ConsSet`<br>&gt;=C2=A0 =C2=A0 data Set a =3D NilSet | ConsSet=
 a (Set a) =C2=A0deriving (Eq, Show, Read)<br><br><br><div class=3D"gmail_q=
uote"><div dir=3D"ltr" class=3D"gmail_attr">On Wed, 30 Jun 2021 at 19:00, A=
nthony Clayden &lt;<a href=3D"mailto:[email protected]">anthony.d=
[email protected]</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote=
" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);=
padding-left:1ex"><div dir=3D"ltr">Ok thank you for the feedback, I get the=
 message not to re-purpose currently valid syntax.<div><br></div><div><br><=
/div><div>&gt;=C2=A0 =C2=A0 [x, y ,: ys ]=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0-- ? not currently valid</div><div><br></div><div>We could make list=
 syntax work harder</div><div><br></div><div>&gt;=C2=A0 =C2=A0 [x, y ,:+ ys=
 ]</div><div><br></div><div>Means desugar the commas by applying constructo=
r `:+` instead of `:`. That could be in general any constructor starting `:=
`.</div><div><br></div><div>Or indeed could be a pattern synonym starting `=
:`, which is a &#39;smart constructor&#39; to build the list maintaining so=
me invariant.</div><div><br></div><div><br></div></div>
</blockquote></div></div></div>

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