1 PLAI Scheme

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► | 1 | PLAI Scheme

1.1 | Defining Types: define-type
1.2 | Deconstructing Data Structures: type-case
1.3 | Testing Infrastructure

On this page:

1.1`` Defining Types: define-type
define-type
1.2`` Deconstructing Data Structures: type-case
type-case
1.3`` Testing Infrastructure
test
test/ pred
error
test/ exn
test/ regexp
1.3.1`` Test Flags
abridged-test-output
plai-catch-test-exn
halt-on-errors
print-only-errors
test-inexact-epsilon
plai-ignore-exn-strings
plai-all-test-results

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### 1`` PLAI Scheme

#lang plai

PLAI Scheme is derived from the scheme language. In addition, it includes the define-type and type-case forms and testing support.

#### 1.1`` Defining Types: define-type

> syntax (define-type type-id variant ...)
> variant |  | = |  | (variant-id (field-id contract-expr) ...)

Defines the datatype type-id. A constructor variant-id is defined for each variant. Each constructor takes an argument for each field of its variant.

The value of each field is checked by its associated contract-expr. A contract-expr may be an arbitrary predicate or a contract.

In addition to the contructors, a define-type expression also defines:

- a predicate type-id? that returns true for instances of the datatype, and false for any other value,

- for each variant, a predicate variant-id? that returns true when applied to a value of the same variant and false for any other value,

- for each field of each variant, an accessor variant-id-field-id that returns the value of the field, and

- for each field of each variant, a mutator set-variant-id-field-id! that set the value of the field.

#### 1.2`` Deconstructing Data Structures: type-case

> > syntax
> > (type-case datatype-id expr
> > branch ...)
> branch |  | = |  | (variant-id (field-id ...) result-expr ...)
> |  | | |  | (else result-expr ...)

Branches on the datatype instance produced by expr, which must be an instance of datatype-id (previously defined with define-type) Each branch extracts the values of the fields, and binds them to field-id ....

If a branch is not specified for each variant, you may use an else branch to create a catch-all branch. An else branch must be the last branch in the sequence of branches. type-case signals a compile-time error if all variants are not covered and the else branch is missing. Similarly, type-case signals a compile-time error if an else branch is unreachable because a branch exists for all variants.

#### 1.3`` Testing Infrastructure

PLAI Scheme provides the following syntactic forms for testing.

> syntax (test result-expr expected-expr)

If result-expr and expected-expr evaluate to the same value, result-value, the test prints the following expression:

(good result-expr result-value expected-value location).

If they do not evaluate to the same value, the test prints

(bad result-expr result-value expected-value location).

If evaluating result-expr signals an error, the test prints

(exception result-expr exception-message <no-expected-value> location)

If evaluating expected-expr signals an error, the test prints

(pred-exception result-expr exception-message <no-expected-value> location)

If the printout begins with good, then it is printed to (current-output-port); otherwise it is printed to (current-error-port).

> syntax (test/pred result-expr pred?)

Similar to test, but instead of supplying an expected value, the predicate pred? is applied to result-expr.

If evaluating pred? signals an error, the test prints

(pred-exception result-expr exception-message <no-expected-value> location)

The syntax of pred? is considered expected-value for the purposes of test reporting.

> value error : procedure?

Like scheme’s scheme:error, but generates exceptions that are caught by test/exn.

> syntax (test/exn result-expr error-message)

This test succeeds if the expression evaluates to a call to error. Moreover, the error message contained in the exception must contain the string error-message. Note that test/exn only succeeds if the exception was explicitly raised by the user.

For example, the following test succeeds:

> (test/exn (error "/: division by zero") "by zero")

The error message is "/: division by zero", and "by zero" is a substring of the error message. However, the following test fails:

> (test/exn (/ 25 0) "by zero")

Although the expression raises an exception and the error string contains "by zero", since the error was not explicitly raised by user-written code, the test fails.

The evaluation of error-message is considered expected-value for the purposes of test reporting.

> syntax (test/regexp result-expr error-message-regexp)

This test is similar to test/exn,but the error message is matched against a regular expression instead.

The evaluation of error-message-regexp is considered expected-value for the purposes of test reporting.

##### 1.3.1`` Test Flags

> procedure (abridged-test-output [abridge?]) → void?
> abridge? : boolean? = false

When this flag is set to true, the test forms never prints result-expr or location.

> procedure (plai-catch-test-exn [catch?]) → void?
> catch? : boolean? = true

When this flag is set to true, exceptions from tests will be caught. By default, exceptions are caught.

> procedure (halt-on-errors [halt?]) → void?
> halt? : boolean? = true

This flag determines whether the program immediately halts when a test fails. By default, programs do not halt on failures.

> procedure (print-only-errors [print?]) → void?
> print? : boolean? = true

When this flag is set to true, only tests that fail will be printed. By default, the results of all tests are printed.

> procedure (test-inexact-epsilon epsilon) → void?
> epsilon : number?

When testing inexact values for equality, test permits them to differ by epsilon. The default value of epsilon is 0.01.

> procedure (plai-ignore-exn-strings ignore?) → void?
> ignore? : boolean?

If this flag is set to true, when testing for exceptions with test/exn and test/regexp, the message of the exception is ignored. By default, test/exn and test/regexp only succeed when the message of the exception matches the supplied string or regular expression.

> syntax plai-all-test-results

This variable is the list of all tests that have been run so far, with the most recent test at the head.
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