5585f9823f
@Kha I implemented the syntax for structure/class that we discussed this morning. It is much cleaner. See new tests at `struct2.lean`. I updated the documentation to use it.
58 lines
1.8 KiB
Markdown
58 lines
1.8 KiB
Markdown
# String interpolation
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The `s!` prefix identifies a string literal as an interpolated string.
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An interpolated string is a string literal that might contain interpolation expressions.
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When an interpolated string is resolved to a result string, items with interpolation expressions are
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replaced by the string representations of the expression results. The polymorphic method `toString` is used
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to convert the value into a string.
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String interpolation provides a more readable and convenient syntax to create formatted strings than
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a string composite formatting feature. The following example uses both features to produce the same output:
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```lean
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def name := "John"
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def age := 28
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#eval IO.println s!"Hello, {name}! Are you {age} years old?"
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#eval IO.println ("Hello, " ++ name ++ "! Are you " ++ toString age ++ " years old?")
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-- `println! <interpolated-string>` is a macro for `IO.println s!<interpolated-string>`
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#eval println! "Hello, {name}! Are you {age} years old?"
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```
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# Structure of an interpolated string
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To identify a string literal as an interpolated string, prepend it with `s!`.
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Terms inside braces `{}` are ordinary expressions whose type implements the type class `ToString`.
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To include a curly brace `{` in your interpolated string, you must escape it using `\{`.
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You can nest interpolated strings inside interpolated strings.
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```lean
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def vals := [1, 2, 3]
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#eval IO.println s!"\{ vals := {vals} }"
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#eval IO.println s!"variables: {vals.map (fun i => s!"x_{i}")}"
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```
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# `ToString` instances
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You can define a `ToString` instance for your own datatypes.
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```lean
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structure Person where
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name : String
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age : Nat
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instance : ToString Person where
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toString : Person -> String
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| { name := n, age := v } => s!"\{ name := {n}, age := {v} }"
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def person1 : Person := {
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name := "John"
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age := 28
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}
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#eval println! "person1: {person1}"
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```
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