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From: Michael Hanus <hanus_at_informatik.rwth-aachen.de>

Date: Thu, 22 Oct 1998 13:32:22 +0200

Dear Colleagues,

after a discussion with different people about some syntactical

aspects of Curry (see also Sven Panne's email of Feb. 9),

Herbert Kuchen and I want to propose a slight change in

the syntax for constraints in Curry. The main point is to get

rid of the special syntax for constraints (currently: enclosing

with curly brackets) and consider constraints as expressions

of the special type "Constraint". This simplifies the overall

syntax of Curry and avoids the syntactic conflict of the

current constraint notation and Haskell's record update syntax

(compare Sven Panne's email of Feb. 9). Our detailed proposal

and its consequences are discussed in the following.

1. Constraints are expressions from a syntactic point of view,

i.e., the curly brackets around constraints are omitted.

2. The symbol "=:=" is used for unification, i.e., the unification

of two expressions e1,e2 is written as "e1 =:= e2" instead

of "e1 = e2". The use of "=" for unification was originally

intended to be conform with unification in Prolog but causes

some trouble in the context of a Haskell syntax. On the other

hand, the use of "=:=" for unification can be also seen

as conform with Prolog since the ISO standard uses "=:="

for arithmetic equality (including the evaluation of both sides)

and several Prolog systems uses this symbol for equational

constraints.

3. The concurrent conjunction is written with the infix operator "&"

and "success" denotes the always satisfiable constraint.

4. In expressions of the form "let x1,...,xn free in c",

c must be an expression of type "Constraint".

5. In conditional rules of the form "l | c = r", the condition c

is an expression of type "Constraint".

In order to be compatible with Haskell, we allow the following

syntactic sugar:

- a rule of the form "l | b = r" where b is of type "Bool" is

considered as an abbreviation for "l | b=:=True = r"

- a rule with multiple guards of Boolean type, i.e.,

l | b1 = r1

| b2 = r2

...

| bn = rn

where each bi has type "Bool", is considered as an abbreviation for

l = if b1 then r1 else

if b2 then r2 else

...

if bn then rn else undefined

(where "undefined" is a non-reducible function).

This means that Booleans guards have a sequential reading

rather than a parallel one which is reasonable for constraints.

Note that this has a subtle consequence for the type inference.

In contrast to Miranda or Haskell, the type inferencer cannot

assume that the guard has always type Bool, but it must assume

an unknown type and performs the inference. If this unknown

type is not further restricted (i.e., remains polymorphic),

it is specialized to "Constraint", otherwise it must be

either "Constraint" or "Bool". Although this could lead

to another typing than in Haskell, it seems to be not a

problem in practice (deduced from many examples that I checked).

Furthermore, one could always add a type signature.

5. Defaults for flex/rigid: Functions of type "Constraint"

(previously: Bool) are flexible, all others are rigid by default.

From the different examples I have, this default is reasonable

(compare also my email of May 6) and also advantageous in

combination with encapsulated search.

If nobody has serious objections against these changes,

I'll include it in the next version of the Curry report.

The necessary changes for the already existing implementations

should be quite small.

Best regards,

Michael

Received on Thu Oct 22 1998 - 14:36:00 CEST

Date: Thu, 22 Oct 1998 13:32:22 +0200

Dear Colleagues,

after a discussion with different people about some syntactical

aspects of Curry (see also Sven Panne's email of Feb. 9),

Herbert Kuchen and I want to propose a slight change in

the syntax for constraints in Curry. The main point is to get

rid of the special syntax for constraints (currently: enclosing

with curly brackets) and consider constraints as expressions

of the special type "Constraint". This simplifies the overall

syntax of Curry and avoids the syntactic conflict of the

current constraint notation and Haskell's record update syntax

(compare Sven Panne's email of Feb. 9). Our detailed proposal

and its consequences are discussed in the following.

1. Constraints are expressions from a syntactic point of view,

i.e., the curly brackets around constraints are omitted.

2. The symbol "=:=" is used for unification, i.e., the unification

of two expressions e1,e2 is written as "e1 =:= e2" instead

of "e1 = e2". The use of "=" for unification was originally

intended to be conform with unification in Prolog but causes

some trouble in the context of a Haskell syntax. On the other

hand, the use of "=:=" for unification can be also seen

as conform with Prolog since the ISO standard uses "=:="

for arithmetic equality (including the evaluation of both sides)

and several Prolog systems uses this symbol for equational

constraints.

3. The concurrent conjunction is written with the infix operator "&"

and "success" denotes the always satisfiable constraint.

4. In expressions of the form "let x1,...,xn free in c",

c must be an expression of type "Constraint".

5. In conditional rules of the form "l | c = r", the condition c

is an expression of type "Constraint".

In order to be compatible with Haskell, we allow the following

syntactic sugar:

- a rule of the form "l | b = r" where b is of type "Bool" is

considered as an abbreviation for "l | b=:=True = r"

- a rule with multiple guards of Boolean type, i.e.,

l | b1 = r1

| b2 = r2

...

| bn = rn

where each bi has type "Bool", is considered as an abbreviation for

l = if b1 then r1 else

if b2 then r2 else

...

if bn then rn else undefined

(where "undefined" is a non-reducible function).

This means that Booleans guards have a sequential reading

rather than a parallel one which is reasonable for constraints.

Note that this has a subtle consequence for the type inference.

In contrast to Miranda or Haskell, the type inferencer cannot

assume that the guard has always type Bool, but it must assume

an unknown type and performs the inference. If this unknown

type is not further restricted (i.e., remains polymorphic),

it is specialized to "Constraint", otherwise it must be

either "Constraint" or "Bool". Although this could lead

to another typing than in Haskell, it seems to be not a

problem in practice (deduced from many examples that I checked).

Furthermore, one could always add a type signature.

5. Defaults for flex/rigid: Functions of type "Constraint"

(previously: Bool) are flexible, all others are rigid by default.

From the different examples I have, this default is reasonable

(compare also my email of May 6) and also advantageous in

combination with encapsulated search.

If nobody has serious objections against these changes,

I'll include it in the next version of the Curry report.

The necessary changes for the already existing implementations

should be quite small.

Best regards,

Michael

Received on Thu Oct 22 1998 - 14:36:00 CEST

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