# Expressions & Patterns

## Expressions

In Pinafore, the main program is a single expression. These are the different kind of expressions:

*string literals*:`"abc\ndef"`

. Use backslash for certain escapes.*numeric literals*:`34.5`

,`~34.5`

. The first is a rational, the second is an inexact number (internally represented as a double).*names*:`x`

. These are bound by declarators (e.g`let`

) and patterns in function bindings (`fn`

and`match`

).*functions*:`fn x => x + 2 * x`

,`match Just x => x; None => 0; end`

. These are constructed by`fn`

(which have one “case”, matching a pattern to an expression) or by`match`

(with multiple cases)*if expressions*:`if t then "yes" else "no"`

.*declarated expressions*:`let x = 3 in x + x`

,`with N in x`

. The declarator modifies the expressions to pass declarations.*do-notation*:`do x <- f; return x + x; end`

. This notation makes it easier to work with monad-like types.*applicative notation*:`{.List %x + %y}`

. This notation makes it easier to work with applicative-like types.*function applications*:`sort mylist`

. This is applying an expression (the function) to an expression (the argument).*list constructors*:`[1,2,3]`

.*tuple constructors*:`(1,"xyz",())`

. Note that there are no tuple types larger than pairs, so this is equivalent to`(1,("xyz",()))`

.

## Patterns

Patterns are for matching against values in function cases (`fn`

and `match`

) and in `let`

-bindings.

An underscore matches everything.

A lower-case name matches everything, and binds to the matched value.

A capitalised name (followed by patterns) matches a data constructor.

The form

`P@Q`

matches when both patterns`P`

and`Q`

match.The form

`P : T`

matches when`P`

matches. Pinafore will reject this if pattern`P`

cannot be subsumed to type`T`

.The form

`P :? T`

matches when the value can be dynamically cast to`T`

, and`P`

matches that.The form

`P as N`

matches when`P`

matches. Names matched in`P`

will be put in namespace`N`

.