Basic Haskell: interaction with the Hugs interpreter

 

 Functional Programming in Haskell
3rd CCSC Northwest Conference • Fall 2001

Basic Haskell: interaction with the Hugs interpreter
bullet A calculator-style interpretive interface
We'll use the Hugs interpreter: it provides an interactive command-line on which user expressions can be evaluated and printed 
> 2 +3 * 5
17

> reverse "Hello, world!"
"!dlrow ,olleH"

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 Functional Programming in Haskell
3rd CCSC Northwest Conference • Fall 2001

Basic Haskell: interaction with the Hugs interpreter
bullet A calculator-style interpretive interface
bullet The "$$" short-cut
We can use a short-cut to refer to the last expression evaluated; this allows for a convenient exploratory paradigm 
> 2 + 3 * 5
17
> 2 ^ $$
131072
> $$ `mod` 3
2

> replicate 3 "hello"
["hello","hello","hello"]
> concat $$
"hellohellohello"
> length $$
15

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 Functional Programming in Haskell
3rd CCSC Northwest Conference • Fall 2001

Basic Haskell: interaction with the Hugs interpreter
bullet A calculator-style interpretive interface
bullet The "$$" short-cut
bullet Interpreter response versus printed output
The response we receive from Hugs is formatted to allow for easy re-entry; we can also print output in a more traditional format 
> "hello, world!"
"hello, world!"

> putStr $$
hello, world!

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 Functional Programming in Haskell
3rd CCSC Northwest Conference • Fall 2001

Basic Haskell: interaction with the Hugs interpreter
bullet A calculator-style interpretive interface
bullet The "$$" short-cut
bullet Interpreter response versus printed output
bullet Interpreter commands
Hugs also supports a number of "meta-level" commands at the prompt, including the following useful ones:
  •  :t  -- reports the type of an expression
  •  :s +s  -- turns on resource statistics
  •  :n  -- looks up names based on patterns
  •  :l  -- loads program files to provide definitions
  •  :q  -- quits the interpreter

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